The Windows Vista TCP/IP stack came packed with a multitude of new features, one being Receive Window Auto-Tuning. The TCP receive window size is the amount of data that a TCP receiver allows a TCP sender to send before having to wait for an acknowledgment. After the connection is established, the receive window size is advertised in each TCP segment. Advertising the maximum amount of data that the sender can send is a receiver-side flow control mechanism that prevents the sender from sending data that the receiver cannot store. A sending host can only send at a maximum the amount of data advertised by the receiver before waiting for an acknowledgment and a receive window size update.
While the intention of this feature is to enhance user experience, this has certainly not been the case for me. Rather than improving network transfers it caped mine at 50kb/s. To disable Receive Window Auto-Tuning open up an elevated command prompt and issue the command:
netsh interface tcp set global autotuninglevel=disable