There has to be something you've installed or obtained unwillingly that is causing the problem... All 4 of the pc's I have Windows 7 running on are still running just as good as the day I installed them.. and I've installed alot of apps and a few games on them.. none of which have caused any decrease in the performance of Win 7 at all...
Odd problem your having... hopefully it can be diagnosed and solved so you can get back to enjoying the beta rather than be p!ssed at it..