I dun no, is it? you tell us...
Personally, at different points depending on which system I am sitting in front of on my network, I load WB or DeskTop X for my user account. Otherwise the system has the same boring MS look and feel, I also run WinStep FULL and Hoverdesk and Aston on diffrent system depending on the box and what I feel like at the moment.
It's a personal choice, and if it happens to slow your system down, then you probably have other issues beyond WindowBlinds... As a matter of fact, why don't you run the XPBench.exe benchmark program without WB loaded, take a screen shot, and then run it again with WB loaded and take a screen shot. You might find some interesting results and it might also give you an idea of why there is a slow down.