I have the same problem with mine: the Apple TV 2 is so slow to start streaming movies I initially thought it wasn't working at all. I'd select a movie, and after many minutes of no result I'd cancel and try another. Turns out it's just so slow I though it wasn't functional.
My setup: movies on iMac (current model 27" i7, OS X 10.6.5), stock internal drive, Airport Express running 7.4.2 in 201.11n only mode 2.4 GHz), no other computers or devices on the network.
I tried swapping in a wired router and connected everything via Ethernet, but it made no difference.
Also tried selecting a movie, waiting the incredibly long time for it to start streaming, and then cancelling out and starting all over with the same movie (to see if it caches). The problem persists.
I wondered if it was the size of my movie library. I have about 230 movies in iTunes. So I tried leaving the Apple TV in "browse your movies" mode overnight to see if it needed time to prefetch things from the movie library. That didn't work either.
I've tried a couple dozen movies from various sources (home movie from iMovie, DVD ripped by Handbrake using both "Apple TV" and "Apple Universal" settings, and Ripit), without any change. I'm going to try the many-chapters idea and made a movie with tons of chapters to see if that helps (will report back).