I am experiencing the same difficulty but my video in also standard definition. Estimated AVI conversion time is 6 hours for only half (15min) of video. I suspect that final cut may be set to convert in background or something but I'm not sure. Hope you find a solution to this problem.
You would be better off exporting a master file to QuickTime with Current Settings & Self Contained.
Then use Compressor to make the AVI files from that.
Compressor can be set up to use all the cores of your CPU: