As I understands it - This has nothing to do with Your iMac - Any Mac would take the same time.
May be OS and support on the Window-PC platform handles this better.
• .avi - is no specific codec (file format) but a container and the codec in this might not be one that iDVD can directly use (encode to .mpeg2 as on DVD disks)
So either it just can't do it - or -
iDVD has to back-encode to an usably codec - THIS IS WHAT I THINK HAS HAPPENED
This back-encoding can take serious time - AND the final result usually is far from Good.
• find out in what codec the .avi file is (I use QuickTime and in this inspector to read what codec .avi contains) (.mov is also a container)
• import it into a program that can transcode to a better codec for iDVD - streamingDV or AIC (I GUESS)
• then use this in iDVD
Yours Bengt W