Set TCPNODELAY as default.

akka-actor/src/main/scala/akka/io/Tcp.scala

@@ -50,6 +50,8 @@ object Tcp extends ExtensionKey[TcpExt] {
      * [[akka.io.Inet.SocketOption]] to enable or disable TCP_NODELAY
      * (disable or enable Nagle's algorithm)
      *
+     * Please note, that TCP_NODELAY is enabled by default.
+     *
      * For more information see [[java.net.Socket.setTcpNoDelay]]
      */
     case class TcpNoDelay(on: Boolean) extends SocketOption {

akka-actor/src/main/scala/akka/io/TcpConnection.scala

@@ -109,6 +109,8 @@ private[io] abstract class TcpConnection(val channel: SocketChannel,
 
   /** used in subclasses to start the common machinery above once a channel is connected */
   def completeConnect(commander: ActorRef, options: immutable.Traversable[SocketOption]): Unit = {
+    // Turn off Nagle's algorithm by default
+    channel.socket.setTcpNoDelay(true)
     options.foreach(_.afterConnect(channel.socket))
 
     commander ! Connected(