Well, for starters it wasn't designed for Vista, which may mean the game is running in some kind of compatibility mode - these never help a game to run well compared to the actual native OS that is being emulated.

Second, Vista is brand new. A new OS often means you get a lot of behavior that is just a little off, especially as companies try to make their graphical and audio solutions work flawlessly with the new platform. So while it'll say it supports everything under the sun, it often takes a little while and several driver versions before things really begin to work as well as they used to with the old OS.

Third, Vista is probably not completely optimized yet. You don't think MS has been issuing all the XP hotfixes for fun, do you? An OS is always a work in progress, so what you're noticing may simply be that Vista has not been sufficiently patched yet to work smoothly and quickly.

After numerous sad upgrade experiences, I've determined a rule: You don't want anything to do with a Microsoft operating system until it gets its first service pack. That seems to be roughly the point where it becomes more useful than nuisance, and where most things you try to do will actually work correctly. While they seem to be making some little progress in this area, I severely doubt that Vista will break form significantly and really be ready prior to SP1.

So for the moment I plan not to look at Vista until it has a service pack out, and frankly haven't heard any glowing reviews yet either that would make me reconsider that viewpoint...