RWIN auto-tuning allows Windows to make better use of high network bandwidth to download / stream data faster, but it is not enabled by default for HTTP connections. This is because the HTTP protocol usually travels over the Internet, where bad routers not supporting RWIN auto-tuning can cause a slow or dropped connections.
It is possible to enable RWIN auto-tuning for Internet Explorer with this DWORD value:
More Info MS KB947239
[HKEY_LOCAL_MACHINE \Software \Microsoft \Windows \CurrentVersion \InternetSettings]
TcpAutotuning = 1