Leopard is setting RWIN too large!!!
Default TCP Receive Window (RWIN) = 524280
RWIN Scaling (RFC1323) = 3 bits (scale factor of 6)
Unscaled TCP Receive Window = 65535
This is way off normal, and changing the RWIN in terminal has no effect. It stays at 524280, which is too large except for an extremely fast connection. Nothing I could do in terminal would set the RWIN to a correct range.
I finally turned scaling off (RFC1323=0) which set my RWIN to 65535, but that is a bit low for my cable connection. There does not seem to be a way to get any other size.
Mac OS X (10.5)