Conversion speeds depend on the speed of your computer for the most part. But Nero isn’t the best conversion program anyway.
To go from avi all the way to dvd with free programs, you might try FAVC or DVDFlick.
If you absolutely want the fastest encoder, buy CCE Basic, read a great deal of information on how to use it and get an authoring program to go with it.
But if you are driving a Volkswagon bug of a computer, you can’t expect to drag race with it.