diff --git a/actix-http/CHANGES.md b/actix-http/CHANGES.md
index 4ec981174..ccfdc81d6 100644
--- a/actix-http/CHANGES.md
+++ b/actix-http/CHANGES.md
@@ -6,6 +6,11 @@
 
 * Update `actix-connect` and `actix-tls` dependency to 2.0.0-alpha.1
 
+* Change default initial window size and connection window size for HTTP2 to 2MB and 1MB respectively to improve download speed for awc when downloading large objects.
+
+* client::Connector accepts initial_window_size and initial_connection_window_size HTTP2 configuration
+
+* client::Connector allowing to set max_http_version to limit HTTP version to be used
 
 ## [2.0.0-alpha.1] - 2020-02-27
 
diff --git a/actix-http/src/client/config.rs b/actix-http/src/client/config.rs
new file mode 100644
index 000000000..c86c697a2
--- /dev/null
+++ b/actix-http/src/client/config.rs
@@ -0,0 +1,39 @@
+use std::time::Duration;
+
+// These values are taken from hyper/src/proto/h2/client.rs
+const DEFAULT_H2_CONN_WINDOW: u32 = 1024 * 1024 * 2; // 2mb
+const DEFAULT_H2_STREAM_WINDOW: u32 = 1024 * 1024; // 1mb
+
+/// Connector configuration
+#[derive(Clone)]
+pub(crate) struct ConnectorConfig {
+    pub(crate) timeout: Duration,
+    pub(crate) conn_lifetime: Duration,
+    pub(crate) conn_keep_alive: Duration,
+    pub(crate) disconnect_timeout: Option<Duration>,
+    pub(crate) limit: usize,
+    pub(crate) conn_window_size: u32,
+    pub(crate) stream_window_size: u32,
+}
+
+impl Default for ConnectorConfig {
+    fn default() -> Self {
+        Self {
+            timeout: Duration::from_secs(1),
+            conn_lifetime: Duration::from_secs(75),
+            conn_keep_alive: Duration::from_secs(15),
+            disconnect_timeout: Some(Duration::from_millis(3000)),
+            limit: 100,
+            conn_window_size: DEFAULT_H2_CONN_WINDOW,
+            stream_window_size: DEFAULT_H2_STREAM_WINDOW,
+        }
+    }
+}
+
+impl ConnectorConfig {
+    pub(crate) fn no_disconnect_timeout(&self) -> Self {
+        let mut res = self.clone();
+        res.disconnect_timeout = None;
+        res
+    }
+}
diff --git a/actix-http/src/client/connector.rs b/actix-http/src/client/connector.rs
index 055d4276d..adb88bbed 100644
--- a/actix-http/src/client/connector.rs
+++ b/actix-http/src/client/connector.rs
@@ -11,6 +11,7 @@ use actix_service::{apply_fn, Service};
 use actix_utils::timeout::{TimeoutError, TimeoutService};
 use http::Uri;
 
+use super::config::ConnectorConfig;
 use super::connection::Connection;
 use super::error::ConnectError;
 use super::pool::{ConnectionPool, Protocol};
@@ -48,11 +49,7 @@ type SslConnector = ();
 /// ```
 pub struct Connector<T, U> {
     connector: T,
-    timeout: Duration,
-    conn_lifetime: Duration,
-    conn_keep_alive: Duration,
-    disconnect_timeout: Duration,
-    limit: usize,
+    config: ConnectorConfig,
     #[allow(dead_code)]
     ssl: SslConnector,
     _t: PhantomData<U>,
@@ -71,42 +68,49 @@ impl Connector<(), ()> {
             > + Clone,
         TcpStream,
     > {
-        let ssl = {
-            #[cfg(feature = "openssl")]
-            {
-                use actix_connect::ssl::openssl::SslMethod;
-
-                let mut ssl = OpensslConnector::builder(SslMethod::tls()).unwrap();
-                let _ = ssl
-                    .set_alpn_protos(b"\x02h2\x08http/1.1")
-                    .map_err(|e| error!("Can not set alpn protocol: {:?}", e));
-                SslConnector::Openssl(ssl.build())
-            }
-            #[cfg(all(not(feature = "openssl"), feature = "rustls"))]
-            {
-                let protos = vec![b"h2".to_vec(), b"http/1.1".to_vec()];
-                let mut config = ClientConfig::new();
-                config.set_protocols(&protos);
-                config
-                    .root_store
-                    .add_server_trust_anchors(&actix_tls::rustls::TLS_SERVER_ROOTS);
-                SslConnector::Rustls(Arc::new(config))
-            }
-            #[cfg(not(any(feature = "openssl", feature = "rustls")))]
-            {}
-        };
-
         Connector {
-            ssl,
+            ssl: Self::build_ssl(vec![b"h2".to_vec(), b"http/1.1".to_vec()]),
             connector: default_connector(),
-            timeout: Duration::from_secs(1),
-            conn_lifetime: Duration::from_secs(75),
-            conn_keep_alive: Duration::from_secs(15),
-            disconnect_timeout: Duration::from_millis(3000),
-            limit: 100,
+            config: ConnectorConfig::default(),
             _t: PhantomData,
         }
     }
+
+    // Build Ssl connector with openssl, based on supplied alpn protocols
+    #[cfg(feature = "openssl")]
+    fn build_ssl(protocols: Vec<Vec<u8>>) -> SslConnector 
+    {
+        use actix_connect::ssl::openssl::SslMethod;
+        use bytes::{BufMut, BytesMut};
+
+        let mut alpn = BytesMut::with_capacity(20);
+        for proto in protocols.iter() {
+            alpn.put_u8(proto.len() as u8);
+            alpn.put(proto.as_slice());
+        }
+
+        let mut ssl = OpensslConnector::builder(SslMethod::tls()).unwrap();
+        let _ = ssl
+            .set_alpn_protos(&alpn)
+            .map_err(|e| error!("Can not set alpn protocol: {:?}", e));
+        SslConnector::Openssl(ssl.build())
+    }
+
+    // Build Ssl connector with rustls, based on supplied alpn protocols
+    #[cfg(all(not(feature = "openssl"), feature = "rustls"))]
+    fn build_ssl(protocols: Vec<Vec<u8>>) -> SslConnector 
+    {
+        let mut config = ClientConfig::new();
+        config.set_protocols(&protocols);
+        config
+            .root_store
+            .add_server_trust_anchors(&actix_tls::rustls::TLS_SERVER_ROOTS);
+        SslConnector::Rustls(Arc::new(config))
+    }
+
+    // ssl turned off, provides empty ssl connector
+    #[cfg(not(any(feature = "openssl", feature = "rustls")))]
+    fn build_ssl(_: Vec<Vec<u8>>) -> SslConnector {}
 }
 
 impl<T, U> Connector<T, U> {
@@ -122,11 +126,7 @@ impl<T, U> Connector<T, U> {
     {
         Connector {
             connector,
-            timeout: self.timeout,
-            conn_lifetime: self.conn_lifetime,
-            conn_keep_alive: self.conn_keep_alive,
-            disconnect_timeout: self.disconnect_timeout,
-            limit: self.limit,
+            config: self.config,
             ssl: self.ssl,
             _t: PhantomData,
         }
@@ -146,7 +146,7 @@ where
     /// Connection timeout, i.e. max time to connect to remote host including dns name resolution.
     /// Set to 1 second by default.
     pub fn timeout(mut self, timeout: Duration) -> Self {
-        self.timeout = timeout;
+        self.config.timeout = timeout;
         self
     }
 
@@ -163,12 +163,42 @@ where
         self
     }
 
+    /// Maximum supported http major version
+    /// Supported versions http/1.1, http/2
+    pub fn max_http_version(mut self, val: http::Version) -> Self {
+        let versions = match val {
+            http::Version::HTTP_11 => vec![b"http/1.1".to_vec()],
+            http::Version::HTTP_2 => vec![b"h2".to_vec(), b"http/1.1".to_vec()],
+            _ => unimplemented!("actix-http:client: supported versions http/1.1, http/2"),
+        };
+        self.ssl = Connector::build_ssl(versions);
+        self
+    }
+
+    /// Indicates the initial window size (in octets) for
+    /// HTTP2 stream-level flow control for received data.
+    ///
+    /// The default value is 65,535 and is good for APIs, but not for big objects.
+    pub fn initial_window_size(mut self, size: u32) -> Self {
+        self.config.stream_window_size = size;
+        self
+    }
+
+    /// Indicates the initial window size (in octets) for
+    /// HTTP2 connection-level flow control for received data.
+    ///
+    /// The default value is 65,535 and is good for APIs, but not for big objects.
+    pub fn initial_connection_window_size(mut self, size: u32) -> Self {
+        self.config.conn_window_size = size;
+        self
+    }
+
     /// Set total number of simultaneous connections per type of scheme.
     ///
     /// If limit is 0, the connector has no limit.
     /// The default limit size is 100.
     pub fn limit(mut self, limit: usize) -> Self {
-        self.limit = limit;
+        self.config.limit = limit;
         self
     }
 
@@ -179,7 +209,7 @@ where
     /// exceeds this period, the connection is closed.
     /// Default keep-alive period is 15 seconds.
     pub fn conn_keep_alive(mut self, dur: Duration) -> Self {
-        self.conn_keep_alive = dur;
+        self.config.conn_keep_alive = dur;
         self
     }
 
@@ -189,7 +219,7 @@ where
     /// until it is closed regardless of keep-alive period.
     /// Default lifetime period is 75 seconds.
     pub fn conn_lifetime(mut self, dur: Duration) -> Self {
-        self.conn_lifetime = dur;
+        self.config.conn_lifetime = dur;
         self
     }
 
@@ -202,7 +232,7 @@ where
     ///
     /// By default disconnect timeout is set to 3000 milliseconds.
     pub fn disconnect_timeout(mut self, dur: Duration) -> Self {
-        self.disconnect_timeout = dur;
+        self.config.disconnect_timeout = Some(dur);
         self
     }
 
@@ -216,7 +246,7 @@ where
         #[cfg(not(any(feature = "openssl", feature = "rustls")))]
         {
             let connector = TimeoutService::new(
-                self.timeout,
+                self.config.timeout,
                 apply_fn(self.connector, |msg: Connect, srv| {
                     srv.call(TcpConnect::new(msg.uri).set_addr(msg.addr))
                 })
@@ -231,10 +261,7 @@ where
             connect_impl::InnerConnector {
                 tcp_pool: ConnectionPool::new(
                     connector,
-                    self.conn_lifetime,
-                    self.conn_keep_alive,
-                    None,
-                    self.limit,
+                    self.config.no_disconnect_timeout(),
                 ),
             }
         }
@@ -248,7 +275,7 @@ where
             use actix_service::{boxed::service, pipeline};
 
             let ssl_service = TimeoutService::new(
-                self.timeout,
+                self.config.timeout,
                 pipeline(
                     apply_fn(self.connector.clone(), |msg: Connect, srv| {
                         srv.call(TcpConnect::new(msg.uri).set_addr(msg.addr))
@@ -301,7 +328,7 @@ where
             });
 
             let tcp_service = TimeoutService::new(
-                self.timeout,
+                self.config.timeout,
                 apply_fn(self.connector, |msg: Connect, srv| {
                     srv.call(TcpConnect::new(msg.uri).set_addr(msg.addr))
                 })
@@ -316,18 +343,9 @@ where
             connect_impl::InnerConnector {
                 tcp_pool: ConnectionPool::new(
                     tcp_service,
-                    self.conn_lifetime,
-                    self.conn_keep_alive,
-                    None,
-                    self.limit,
-                ),
-                ssl_pool: ConnectionPool::new(
-                    ssl_service,
-                    self.conn_lifetime,
-                    self.conn_keep_alive,
-                    Some(self.disconnect_timeout),
-                    self.limit,
+                    self.config.no_disconnect_timeout(),
                 ),
+                ssl_pool: ConnectionPool::new(ssl_service, self.config),
             }
         }
     }
diff --git a/actix-http/src/client/h2proto.rs b/actix-http/src/client/h2proto.rs
index 69d20752a..2afd2d80b 100644
--- a/actix-http/src/client/h2proto.rs
+++ b/actix-http/src/client/h2proto.rs
@@ -1,11 +1,15 @@
 use std::convert::TryFrom;
+use std::future::Future;
 use std::time;
 
 use actix_codec::{AsyncRead, AsyncWrite};
 use bytes::Bytes;
 use futures_util::future::poll_fn;
 use futures_util::pin_mut;
-use h2::{client::SendRequest, SendStream};
+use h2::{
+    client::{Builder, Connection, SendRequest},
+    SendStream,
+};
 use http::header::{HeaderValue, CONNECTION, CONTENT_LENGTH, TRANSFER_ENCODING};
 use http::{request::Request, Method, Version};
 
@@ -14,6 +18,7 @@ use crate::header::HeaderMap;
 use crate::message::{RequestHeadType, ResponseHead};
 use crate::payload::Payload;
 
+use super::config::ConnectorConfig;
 use super::connection::{ConnectionType, IoConnection};
 use super::error::SendRequestError;
 use super::pool::Acquired;
@@ -185,3 +190,18 @@ fn release<T: AsyncRead + AsyncWrite + Unpin + 'static>(
         }
     }
 }
+
+pub(crate) fn handshake<Io>(
+    io: Io,
+    config: &ConnectorConfig,
+) -> impl Future<Output = Result<(SendRequest<Bytes>, Connection<Io, Bytes>), h2::Error>>
+where
+    Io: AsyncRead + AsyncWrite + Unpin + 'static,
+{
+    let mut builder = Builder::new();
+    builder
+        .initial_window_size(config.stream_window_size)
+        .initial_connection_window_size(config.conn_window_size)
+        .enable_push(false);
+    builder.handshake(io)
+}
diff --git a/actix-http/src/client/mod.rs b/actix-http/src/client/mod.rs
index a45aebcd5..dd1e9b25a 100644
--- a/actix-http/src/client/mod.rs
+++ b/actix-http/src/client/mod.rs
@@ -1,6 +1,7 @@
 //! Http client api
 use http::Uri;
 
+mod config;
 mod connection;
 mod connector;
 mod error;
diff --git a/actix-http/src/client/pool.rs b/actix-http/src/client/pool.rs
index 38a51b558..983396f92 100644
--- a/actix-http/src/client/pool.rs
+++ b/actix-http/src/client/pool.rs
@@ -13,14 +13,16 @@ use actix_utils::{oneshot, task::LocalWaker};
 use bytes::Bytes;
 use futures_util::future::{poll_fn, FutureExt, LocalBoxFuture};
 use fxhash::FxHashMap;
-use h2::client::{handshake, Connection, SendRequest};
+use h2::client::{Connection, SendRequest};
 use http::uri::Authority;
 use indexmap::IndexSet;
 use pin_project::pin_project;
 use slab::Slab;
 
+use super::config::ConnectorConfig;
 use super::connection::{ConnectionType, IoConnection};
 use super::error::ConnectError;
+use super::h2proto::handshake;
 use super::Connect;
 
 #[derive(Clone, Copy, PartialEq)]
@@ -50,20 +52,11 @@ where
     T: Service<Request = Connect, Response = (Io, Protocol), Error = ConnectError>
         + 'static,
 {
-    pub(crate) fn new(
-        connector: T,
-        conn_lifetime: Duration,
-        conn_keep_alive: Duration,
-        disconnect_timeout: Option<Duration>,
-        limit: usize,
-    ) -> Self {
+    pub(crate) fn new(connector: T, config: ConnectorConfig) -> Self {
         ConnectionPool(
             Rc::new(RefCell::new(connector)),
             Rc::new(RefCell::new(Inner {
-                conn_lifetime,
-                conn_keep_alive,
-                disconnect_timeout,
-                limit,
+                config,
                 acquired: 0,
                 waiters: Slab::new(),
                 waiters_queue: IndexSet::new(),
@@ -129,6 +122,8 @@ where
                     // open tcp connection
                     let (io, proto) = connector.call(req).await?;
 
+                    let config = inner.borrow().config.clone();
+
                     let guard = OpenGuard::new(key, inner);
 
                     if proto == Protocol::Http1 {
@@ -138,7 +133,7 @@ where
                             Some(guard.consume()),
                         ))
                     } else {
-                        let (snd, connection) = handshake(io).await?;
+                        let (snd, connection) = handshake(io, &config).await?;
                         actix_rt::spawn(connection.map(|_| ()));
                         Ok(IoConnection::new(
                             ConnectionType::H2(snd),
@@ -255,10 +250,7 @@ struct AvailableConnection<Io> {
 }
 
 pub(crate) struct Inner<Io> {
-    conn_lifetime: Duration,
-    conn_keep_alive: Duration,
-    disconnect_timeout: Option<Duration>,
-    limit: usize,
+    config: ConnectorConfig,
     acquired: usize,
     available: FxHashMap<Key, VecDeque<AvailableConnection<Io>>>,
     waiters: Slab<
@@ -311,7 +303,7 @@ where
 
     fn acquire(&mut self, key: &Key, cx: &mut Context<'_>) -> Acquire<Io> {
         // check limits
-        if self.limit > 0 && self.acquired >= self.limit {
+        if self.config.limit > 0 && self.acquired >= self.config.limit {
             return Acquire::NotAvailable;
         }
 
@@ -323,10 +315,10 @@ where
             let now = Instant::now();
             while let Some(conn) = connections.pop_back() {
                 // check if it still usable
-                if (now - conn.used) > self.conn_keep_alive
-                    || (now - conn.created) > self.conn_lifetime
+                if (now - conn.used) > self.config.conn_keep_alive
+                    || (now - conn.created) > self.config.conn_lifetime
                 {
-                    if let Some(timeout) = self.disconnect_timeout {
+                    if let Some(timeout) = self.config.disconnect_timeout {
                         if let ConnectionType::H1(io) = conn.io {
                             actix_rt::spawn(CloseConnection::new(io, timeout))
                         }
@@ -338,7 +330,7 @@ where
                         match Pin::new(s).poll_read(cx, &mut buf) {
                             Poll::Pending => (),
                             Poll::Ready(Ok(n)) if n > 0 => {
-                                if let Some(timeout) = self.disconnect_timeout {
+                                if let Some(timeout) = self.config.disconnect_timeout {
                                     if let ConnectionType::H1(io) = io {
                                         actix_rt::spawn(CloseConnection::new(
                                             io, timeout,
@@ -372,7 +364,7 @@ where
 
     fn release_close(&mut self, io: ConnectionType<Io>) {
         self.acquired -= 1;
-        if let Some(timeout) = self.disconnect_timeout {
+        if let Some(timeout) = self.config.disconnect_timeout {
             if let ConnectionType::H1(io) = io {
                 actix_rt::spawn(CloseConnection::new(io, timeout))
             }
@@ -381,7 +373,7 @@ where
     }
 
     fn check_availibility(&self) {
-        if !self.waiters_queue.is_empty() && self.acquired < self.limit {
+        if !self.waiters_queue.is_empty() && self.acquired < self.config.limit {
             self.waker.wake();
         }
     }
@@ -480,6 +472,7 @@ where
                         tx,
                         this.inner.clone(),
                         this.connector.call(connect),
+                        inner.config.clone(),
                     );
                 }
             }
@@ -506,6 +499,7 @@ where
     >,
     rx: Option<oneshot::Sender<Result<IoConnection<Io>, ConnectError>>>,
     inner: Option<Rc<RefCell<Inner<Io>>>>,
+    config: ConnectorConfig,
 }
 
 impl<F, Io> OpenWaitingConnection<F, Io>
@@ -518,6 +512,7 @@ where
         rx: oneshot::Sender<Result<IoConnection<Io>, ConnectError>>,
         inner: Rc<RefCell<Inner<Io>>>,
         fut: F,
+        config: ConnectorConfig,
     ) {
         actix_rt::spawn(OpenWaitingConnection {
             key,
@@ -525,6 +520,7 @@ where
             h2: None,
             rx: Some(rx),
             inner: Some(inner),
+            config,
         })
     }
 }
@@ -594,7 +590,7 @@ where
                     )));
                     Poll::Ready(())
                 } else {
-                    *this.h2 = Some(handshake(io).boxed_local());
+                    *this.h2 = Some(handshake(io, this.config).boxed_local());
                     self.poll(cx)
                 }
             }
diff --git a/awc/CHANGES.md b/awc/CHANGES.md
index d410ae514..1d6e03708 100644
--- a/awc/CHANGES.md
+++ b/awc/CHANGES.md
@@ -1,5 +1,11 @@
 # Changes
 
+## [NEXT]
+
+* ClientBuilder accepts initial_window_size and initial_connection_window_size HTTP2 configuration
+
+* ClientBuilder allowing to set max_http_version to limit HTTP version to be used
+
 ## [1.0.1] - 2019-12-15
 
 * Fix compilation with default features off
diff --git a/awc/src/builder.rs b/awc/src/builder.rs
index 7bd0171ec..2b2e5df9f 100644
--- a/awc/src/builder.rs
+++ b/awc/src/builder.rs
@@ -4,11 +4,11 @@ use std::fmt;
 use std::rc::Rc;
 use std::time::Duration;
 
-use actix_http::client::{Connect, ConnectError, Connection, Connector};
-use actix_http::http::{header, Error as HttpError, HeaderMap, HeaderName};
+use actix_http::client::{Connect as HttpConnect, ConnectError, Connection, Connector};
+use actix_http::http::{header, Error as HttpError, HeaderMap, HeaderName, self};
 use actix_service::Service;
 
-use crate::connect::ConnectorWrapper;
+use crate::connect::{ConnectorWrapper, Connect};
 use crate::{Client, ClientConfig};
 
 /// An HTTP Client builder
@@ -16,10 +16,15 @@ use crate::{Client, ClientConfig};
 /// This type can be used to construct an instance of `Client` through a
 /// builder-like pattern.
 pub struct ClientBuilder {
-    config: ClientConfig,
     default_headers: bool,
     allow_redirects: bool,
     max_redirects: usize,
+    max_http_version: Option<http::Version>,
+    stream_window_size: Option<u32>,
+    conn_window_size: Option<u32>,
+    headers: HeaderMap,
+    timeout: Option<Duration>,
+    connector: Option<RefCell<Box<dyn Connect>>>,
 }
 
 impl Default for ClientBuilder {
@@ -34,25 +39,24 @@ impl ClientBuilder {
             default_headers: true,
             allow_redirects: true,
             max_redirects: 10,
-            config: ClientConfig {
-                headers: HeaderMap::new(),
-                timeout: Some(Duration::from_secs(5)),
-                connector: RefCell::new(Box::new(ConnectorWrapper(
-                    Connector::new().finish(),
-                ))),
-            },
+            headers: HeaderMap::new(),
+            timeout: Some(Duration::from_secs(5)),
+            connector: None,
+            max_http_version: None,
+            stream_window_size: None,
+            conn_window_size: None,
         }
     }
 
     /// Use custom connector service.
     pub fn connector<T>(mut self, connector: T) -> Self
     where
-        T: Service<Request = Connect, Error = ConnectError> + 'static,
+        T: Service<Request = HttpConnect, Error = ConnectError> + 'static,
         T::Response: Connection,
         <T::Response as Connection>::Future: 'static,
         T::Future: 'static,
     {
-        self.config.connector = RefCell::new(Box::new(ConnectorWrapper(connector)));
+        self.connector = Some(RefCell::new(Box::new(ConnectorWrapper(connector))));
         self
     }
 
@@ -61,13 +65,13 @@ impl ClientBuilder {
     /// Request timeout is the total time before a response must be received.
     /// Default value is 5 seconds.
     pub fn timeout(mut self, timeout: Duration) -> Self {
-        self.config.timeout = Some(timeout);
+        self.timeout = Some(timeout);
         self
     }
 
     /// Disable request timeout.
     pub fn disable_timeout(mut self) -> Self {
-        self.config.timeout = None;
+        self.timeout = None;
         self
     }
 
@@ -79,6 +83,31 @@ impl ClientBuilder {
         self
     }
 
+    /// Maximum supported http major version
+    /// Supported versions http/1.1, http/2
+    pub fn max_http_version(mut self, val: http::Version) -> Self {
+        self.max_http_version = Some(val);
+        self
+    }
+
+    /// Indicates the initial window size (in octets) for
+    /// HTTP2 stream-level flow control for received data.
+    ///
+    /// The default value is 65,535 and is good for APIs, but not for big objects.
+    pub fn initial_window_size(mut self, size: u32) -> Self {
+        self.stream_window_size = Some(size);
+        self
+    }
+
+    /// Indicates the initial window size (in octets) for
+    /// HTTP2 connection-level flow control for received data.
+    ///
+    /// The default value is 65,535 and is good for APIs, but not for big objects.
+    pub fn initial_connection_window_size(mut self, size: u32) -> Self {
+        self.conn_window_size = Some(size);
+        self
+    }
+
     /// Set max number of redirects.
     ///
     /// Max redirects is set to 10 by default.
@@ -106,7 +135,7 @@ impl ClientBuilder {
         match HeaderName::try_from(key) {
             Ok(key) => match value.try_into() {
                 Ok(value) => {
-                    self.config.headers.append(key, value);
+                    self.headers.append(key, value);
                 }
                 Err(e) => log::error!("Header value error: {:?}", e),
             },
@@ -140,7 +169,27 @@ impl ClientBuilder {
 
     /// Finish build process and create `Client` instance.
     pub fn finish(self) -> Client {
-        Client(Rc::new(self.config))
+        let connector = if let Some(connector) = self.connector {
+            connector
+        } else {
+            let mut connector = Connector::new();
+            if let Some(val) = self.max_http_version {
+                connector = connector.max_http_version(val)
+            };
+            if let Some(val) = self.conn_window_size {
+                connector = connector.initial_connection_window_size(val)
+            };
+            if let Some(val) = self.stream_window_size {
+                connector = connector.initial_window_size(val)
+            };
+            RefCell::new(Box::new(ConnectorWrapper(connector.finish())) as Box<dyn Connect>)
+        };
+        let config = ClientConfig {
+            headers: self.headers,
+            timeout: self.timeout,
+            connector,
+        };
+        Client(Rc::new(config))
     }
 }
 
@@ -153,7 +202,6 @@ mod tests {
         let client = ClientBuilder::new().basic_auth("username", Some("password"));
         assert_eq!(
             client
-                .config
                 .headers
                 .get(header::AUTHORIZATION)
                 .unwrap()
@@ -165,7 +213,6 @@ mod tests {
         let client = ClientBuilder::new().basic_auth("username", None);
         assert_eq!(
             client
-                .config
                 .headers
                 .get(header::AUTHORIZATION)
                 .unwrap()
@@ -180,7 +227,6 @@ mod tests {
         let client = ClientBuilder::new().bearer_auth("someS3cr3tAutht0k3n");
         assert_eq!(
             client
-                .config
                 .headers
                 .get(header::AUTHORIZATION)
                 .unwrap()
diff --git a/awc/tests/test_connector.rs b/awc/tests/test_connector.rs
new file mode 100644
index 000000000..8b295cda7
--- /dev/null
+++ b/awc/tests/test_connector.rs
@@ -0,0 +1,61 @@
+#![cfg(feature = "openssl")]
+use actix_http::HttpService;
+use actix_http_test::test_server;
+use actix_service::{map_config, ServiceFactory};
+use actix_web::http::Version;
+use actix_web::{dev::AppConfig, web, App, HttpResponse};
+use open_ssl::ssl::{SslAcceptor, SslConnector, SslFiletype, SslMethod, SslVerifyMode};
+
+fn ssl_acceptor() -> SslAcceptor {
+    // load ssl keys
+    let mut builder = SslAcceptor::mozilla_intermediate(SslMethod::tls()).unwrap();
+    builder
+        .set_private_key_file("../tests/key.pem", SslFiletype::PEM)
+        .unwrap();
+    builder
+        .set_certificate_chain_file("../tests/cert.pem")
+        .unwrap();
+    builder.set_alpn_select_callback(|_, protos| {
+        const H2: &[u8] = b"\x02h2";
+        if protos.windows(3).any(|window| window == H2) {
+            Ok(b"h2")
+        } else {
+            Err(open_ssl::ssl::AlpnError::NOACK)
+        }
+    });
+    builder.set_alpn_protos(b"\x02h2").unwrap();
+    builder.build()
+}
+
+#[actix_rt::test]
+async fn test_connection_window_size() {
+    let srv = test_server(move || {
+        HttpService::build()
+            .h2(map_config(
+                App::new().service(
+                    web::resource("/").route(web::to(|| HttpResponse::Ok())),
+                ),
+                |_| AppConfig::default(),
+            ))
+            .openssl(ssl_acceptor())
+            .map_err(|_| ())
+    });
+
+    // disable ssl verification
+    let mut builder = SslConnector::builder(SslMethod::tls()).unwrap();
+    builder.set_verify(SslVerifyMode::NONE);
+    let _ = builder
+        .set_alpn_protos(b"\x02h2\x08http/1.1")
+        .map_err(|e| log::error!("Can not set alpn protocol: {:?}", e));
+
+    let client = awc::Client::build()
+        .connector(awc::Connector::new().ssl(builder.build()).finish())
+        .initial_window_size(100)
+        .initial_connection_window_size(100)
+        .finish();
+
+    let request = client.get(srv.surl("/")).send();
+    let response = request.await.unwrap();
+    assert!(response.status().is_success());
+    assert_eq!(response.version(), Version::HTTP_2);
+}