Act your wage.
I just loaded a new theme yesterday, and I'm almost certain that I see better performance from Windows since doing it. Folders seem to open more quickly, icons seem to refresh faster, etc. I have not restarted my system and so it's not that things have been freshly loaded lately. I've never really seen it stated that certain themes might degrade system performance or vice versa, but it sure seems that this is the case. Possible?