One of the areas we’re putting a lot of work into is XBMC performance – we’ve been a bit shocked on working through some data * to find that the Pi now appears to have more XBMC users than any other platform in the world, bar the PC (we’ve overtaken cracked Apple TV 2s), and we want to make sure you have the best possible experience with the software. (If you’ve started reading this and don’t understand a word of that first paragraph, head over to XBMC’s website to find out what XBMC is, what a media centre is and why you might want one, and then come back here.) Dom Cobley and Ben Avison have been working on the platform for us, and the results so far are pretty impressive: video playback has always been good, but they’ve really tidied up the user experience in the menu in particular, and browsing through your media collection, even if it’s as big as Dom’s, is now much smoother and faster. We’ve seen people online (particularly over on the XBMC forums – and  particularly particularly in response to posts asking for recommendations for cheap XBMC platforms) calling Pi users fanbois, and announcing that the Pi is too laggy to be a real media player

The rest is here:
XBMC performance demo