Does customization slow down your computer? Answer: Not really
http://www.bjorn3d.com/_preview.php?articleID=323
Bjorn3D has put together an article that answers the age old question: Will customizing your Windows PC slow down your computer?
To find out, he loaded up Object Desktop components such as WindowBlinds, ObjectBar, IconPackager, and WinStyles and then put on CursorXP to top it off.
He then ran it thorugha host of benchmarks comparing it to his clean setup. Benchmarks included 3DMark, PCMark, UT2K3, and others.
The result? No discernable performance hit. Check out the article for yourself.
At home on my dinosaure Celeron 333 with only 128 Megs of RAM, I don't run any theme (not even Luna, just plain old Classic), and it does make a difference in speed. But on my work computer (1.4 GHrz w/1G of RAM), it makes no difference in speed whatsoever if I run WB, DX, OB, CX or not. On the other hand WinFX does slow it down.