video conversion can take a long time between formats and your machines hardware spec plays a major role as well as what software your using.
usually a good example of this is a Photoshop file to a jpeg file.
photoshop file retains all the quality and size but the jpeg looks the same just a lot smaller in size.
it takes a while for the computer to guess pixel colours from the raw set. Its got to change the dimensions of the video and then guess all the new colours for each pixel for each cellframe.
possibly in your case large AVI raw file is condensed to mpeg4.
some converters do it quicker then others depending on the alg. and the optimization of the program at hand.
stuff to make it faster.
edit out content from the video to streamline the final length.
use a faster machine
use a faster program to convert it.
turn off slower conversion features in your program.
turn off other software nonsense while your comp does this and leave it alone.
here is an article i found using i movie on the apple platform as an example.
http://docs.info.apple.com/article.html?artnum=302559
while this is hardly the most professional way or the way i would go it might shed some light.
If no-one gives your an answer to a question you have asked, consider:- Is your question clear.- Did you ask nicely.- Are you showing any effort to solve the problem yourself 
