Well you could use the newer ConvertXtoDVD which would be using a newer libavcodec.dll. There may have been some speed ups to ffmpeg since divxtodvd's release. Otherwise you could compile libavcodec with custom cflags for your CPU, but then I believe they made some modifications to the code. Really though you have the wrong forum. This is the DivX/Xvid forum, not the MPEG-2/DVD encoding one.
Get a dual core machine, it really helps. I used to have a single core AMD athlon xp 2800+, 2.2 ghz, a movie would take me about 3 hrs to encode. I have a dual core AM2 AMD 3800+ at 2ghz, and the same movie will prolly encode just under 2 hrs.