SkinBench is a benchmarking application that will use standard Windows API calls with regular windows and window controls (edit boxes, buttons, etc) to evaluate the performance of any skin in use (it will still work without any skin... probably faster.

The idea is that, since it does things a user does (activating windows, moving them around, etc) it should reflect how fast a skin will appear to a user. Obviously, on their own, the numbers mean nothing, so you might want to see how some of the default skins score on your computer for comparison.