Apple recently released a new version of QuickTime (v. 7.2) sporting a number of new features and enhancements.  Among the many enhancements - which include (finally) full screen support in the unregistered version - Apple added generic AVI playback, supporting codecs like Xvid, DivX and an enhanced H.264. On first blush this is a really good thing, but it causes some serious issues for those of us who have added third party codecs to QuickTime in the past to make such things work.  Put simple, the update wipes out all of these third party codecs, and replaces them with the QuickTime standard components.  These components of course provide the same playback, but they also come with their own set of issues. Apparently, with QuickTime 7.2, startup to playback an Xvid/MP3 AVI takes almost 15 seconds, pushing the CPU to 50% the whole time.  When you're used to paging through clips in FrontRow (which fires up QuickTime to play the thumbnails for each film), 15 seconds is a really long time. The solution is to reinstall the codecs required to playback the AVI(s) separately from the vanilla QuickTime components.  Contrary to what I've said previously, I would recommend Perian.  Either I had a buggy install that one time, or it was something weather related.  Either way, I've been using it for several months now and it works great.  You can download it here. Once downloaded, the DMG should prompt you and automatically mount itself. Double click on the "Perian.prefPane" file.  You will want to select "Install for All Users", after which you will need to enter your password (twice).  Once Perian is installed, you will be presented with the preference pane.

Unlike previous versions of Perian, which required manual fiddling with .component files, this latest version doesn't require any more steps to install than what we've already outlined. And that's all there is to it!  QuickTime should now play any movie file at full quality, doing its little startup dance in record time (with minimal strain on the CPU).