It's most likely not the number of items, but instead the size of your posters. If you're using posters significantly bigger that the native poster size, it can seriously impact the navigation speed as the screen scales down the posters.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)