Fastest way to convert .avi files for itunes?
You're not going to find a magic bullet that can dramatically speed up your conversion. You really have two options: either reduce the quality of your target file, or throw more iron at it. Upgrading your hardware (including possibly putting more RAM in your machine) would help a lot.

The bottom line, though, is that the conversion app has to look at the whole file frame by frame and convert it, and that's an O(n) proposition--the time it takes scales linearly with the amount of data to convert, and there just aren't good ways for even a very clever program to shortcut that. Some programs may be fractionally more efficient than others, but in general, the algorithm takes as long as it takes to chew as much data as it has to chew.