I think that the problem is not the mulicamera editing tool. I have worked in HD format and had an hour and a half project and it worked perfect. I think the problem is the kind of file you use in the timeline. In the particular project where I had the problem I was using files that I captured from a tape and files the I had filmed right on to a Hard Drive. When I eliminated the files from the Hard Drive, FCP X started working normal again. So I think it has something to do with the kind of files I was using, even if they were also .mov files.
Marjo