While experimenting with GoLive's Quicktime track editing features, I opened up a movie, saved using Quicktime's Export to MPEG-4 function. GoLive slowed to molasses speeds.

Thinking I was dealing with a GoLive issue, I searched Adobe's boards for mention of slow performance while editing movies. Nothing - although there are plenty of complaints about slow performance in general, which I haven't had problems with. Then I opened a Sorenson version of the same video, and it worked fine. So, I opened the MPEG-4 version in Quicktime Player - same slow performance, outside of GoLive. There must be some key piece that I'm missing that enables rapid scanning through MPEG-4's.