From here http://www.veeam.com/forums/viewtopic.php?f=2&t=4093
It depends and differs from system to system. what you need to know is that microsoft (i guess since vista and server 2008) adjusts the tcp window scaling dynamically with tcp autotuning feature. where this is mostly a good one it can become a bad one on some systems. ive explicitly seen very bad experience with especially high-performance access and rdp access. to get rid of this autotuning feature you have to manually disable it:
netsh, interface, tcp, set global autotuninglevel=disable
I haven’t tried this yet, so let me know if you try it!