Tom's Hardware published an interesting report comparing Windows Vista and Windows XP in various areas.
You can find the report here: http://www.tomshardware.com/2007/01/29/xp-vs-vista/
Personally I am not surprised that Vista performed slower in many applications compared to XP. After all, Vista is quite revolutionary and i guess that it has a lot of extra code to stay compatible with previous applications. I think that Vista performance will improve after software developers start optimizing software for Vista.