Front Row can be slow as molasses sometimes, particularly when browsing the Movies folder. Sometimes it locks up for over a minute before it moves from one item to the next – presumably while it attempts to render a preview of the movie.
After nearly dying of boredom while waiting to watch a movie one time, I decided to rummage around on the Apple forums, and found the solution: The Perian codec I had installed (0.5) needed updating. I grabbed the latest 1.0 version, upgraded, and restarted the Mac. Problem solved. (It still takes a few seconds to enter the Movies folder, but not the 1-minute-plus that it previously took.)
So if you’re gnashing your teeth wondering why Front Row is taking ages to browse the Movies folder – check your codecs!