I have noticed an a situation where/client where network transfer speeds, though not "slow" are much slower from Windows 2003 > Windows 2008 vs. the other way OR from Windows 2003 > Windows 2003 or Windows 2008 to Windows 2008.
Using the simple Lan Speed Test Shareware tool I get between 300-400 Mbps speeds between servers on the same Gig Switch EXCEPT when going from Windows 2003 to Windows 2008 when I only get around 150-200 Mbps, I initially thought it may be improvements in Windows 2008 but that does not fly when you consider I get 400 Mbps between 2 Windows 2003 servers...I have tried disabling SMB along with several netsh commands on one of the Windows 2008 servers (and rebooted) but it does not change the result.
Anyone run into something similar? If I do the test from a specific 2003 server to a specific 2008 serer, get my 150-200Mbps results, I ca then turn around and go on the same exact Windows 2008 server and test back to this same 2003 server and get 2-3X the speed going that way. The Windows 2003 servers are varied with several different kinds of nics so it is an OS level problem I would think