iMovie 8 - import speed of different file types

I recently purchased a new MacBook Pro, and after telling my coworkers how awesome it is, I was asked to take on a couple of corporate video projects. I did public-access-TV video production throughout the 1990s, so this was an ideal opportunity to learn iMovie and iDVD, and see what they're capable of compared to the old Betacam video decks I used to edit with. Suffice it to say that I've been blown away by the power of these two programs. Ah, the masterpieces I could have produced if current technology had been available to me back in the day...

Anway, one project involved compiling a dozen or so short company videos into one long video to play at a conference. Most were on DVDs meant to be played in DVD players (i.e., in VIDEO_TS folders). I used Mac the Ripper to convert them to MP4 files, which I then imported into iMovie. The others were in WMV format. I used the Flip4Mac plug-in to import those. Everything worked fine, but the MP4 files took at four or five times as long to import as the WMV files. For example, a 15 MB, 4-minute WMV clip took about 4 minutes to import, whereas a 15 MB, 4-minute MP4 clip took about 17 minutes. The quality looks about the same to me.

Does anyone know why MP4 files take so much longer to import into iMovie? If I had to guess, I'd say that the MP4 format compresses more heavily than WMV and therefore takes more time to decompress -- which in turn implies that the MP4 clips should be of higher quality than the WMV clips, assuming the source material is of comparable quality. If that's true, then I'm guessing I converted the VIDEO_TS files to higher-quality MP4 files than necessary. Yes? No?