When Citrix sessions are getting ridiculously slow, then this probably is your problem.
Finally found the solution for this problem, Windows Vista has a problem with Citrix ICA Client 10.150 due to a new feature of Vista called "Receive Windows Auto-Tuning Level":
 
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You can disable this feature with the following commandline option: "netsh interface tcp set global autotuninglevel=disabled" (without the quotes).
There seems to be lot of problems with this feature which probably will be is resolved with Vista Service Pack 1.
 
Henny Louwers