I'm just in the process of polishing my mac mini media center a bit (hey, it's 6 in the morning and I'm not tired… what else is there to do? ;D) and ran into some problems with XBMC being really sluggish.
Turns out this was caused by my users privilieges which resulted in XBMC not being able to use the DRI/OpenGL/Whatever fancy renderer and reverting to the software rasterizer.

Solution: I added my user to a bunch of groups (video and wheel might have been the most interesting ones) and now it works like a charm :)