For programmers or people with advanced technical knowledge.
Optimizing your videos, assuming they haven't been purchased from the iTunes Store, is of extreme importance if you stream them to iOS devices, particularly to an Apple TV.
In my previous article, I've shown you some ways of finding out whether a video file is optimized. Check out this article on how optimization needs to be done if you run into a non-optimized video, with, for example, the absolutely excellent Subler.