Windows 7 is shaping up to be a pretty perfomant operating system. I've been ripping CDs in the background, which easily maxes out the processor on my 1.83GHz Pentium M. All the while I've been playing songs and browsing websites with no performance problems. Only just now when I added installing Acrobat Reader on to the top of all that did I notice some slow-downs (slight glitches in music playback).
Now you might say that this is no big deal, an OS should be able to do all of those things. But this is with 512MB of RAM, the afore-mentioned processor, and a crappy laptop HDD. So far I would say the performance is way closer to XP than Vista was on this machine. But the features are way better than XP. I'll try not to get too optimistic, but from what I've seen in the last few hours Windows 7 is going to be a hot item.