Colin and Nick discussed video formats this morning on Twitter. Colin said that “iMovie lags like crazy on H.264 video” which might explain why the transitions in my first commentary didn’t render correctly when I did the voiceover in iMovie, and resulted in the video lagging behind the audio later in the video.
Anyway, I went searching, and found MPEG STREAMCLIP which will convert MP4 files to Apple Intermediate Codec (AIC). A 596MB file became a gigantic 11GB file! I haven’t tried editing a video with transitions yet but hopefully iMovie will perform better.