I just spent way too much time checking out this issue, hopefully someone else can benefit from it.

Overview:
If your are running Windows 7 or Vista and have turned off Aero Glass (or altered Windows Performance Options) to improve your DAW's performance you may have done just the opposite (unless your system has a total crap video card).

Article on Agitated States website (too long to post here) Improving DAW Performance
Image from article:

Article also has a link to performance test app used in getting the above test results.

Best regards

...Steven