It doesn't always give you a performance boost, especially if there is a router that is not RFC compliant somewhere in the 'chain' between your machine and the "series of tubes" (Internet).
Set the following DWORD registry key to "1":
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\InternetSettings\TcpAutotuning
After restarting, Tcp autotuning (RFC 1323) will be applied to HTTP traffic.
You can read more about this
here - "Description of the Receive Window Auto-Tuning feature for HTTP traffic on Windows Vista-based computers"
