Vista isn't slow, it just have higher system requirements than XP. Remember XP is 7 years old by now that is an eternity in computer science.
If you Put Vista And XP on the same Box, then XP would in general run faster then Vista. Thus the conclusion that Vista is slower. Then again, if Microsoft didn't waste so much time with implementing that DRM BS (I mean, a f-ing driver error code for having
too high a resolution? *censored*? and check every single output frame of audio and video for changes to the "tilt bits" of the drivers, it would probably run WAY faster then XP.
That is the main reason I really don't like Vista- the main reason it has the higher requirement isn't necessarily for new functionality, (forgetting aero and such), but rather because there is so much garbage code to prevent us from using functionality it is ridiculous.