Why does QuickTime 7 Pro take a long time to convert movies from one format to another?
Conversion time will vary based on the speed of your computer, how much memory you have available, the size of the movie and complexity of the conversion, and how many other open programs you have. For best conversion time choose single-pass instead of multi-pass encoding. You can do that by choosing File > Export > Options > Settings (under Video) and then selecting Single-pass.