This is a very old question, but apparently so far has not been resolved. I work a lot with creating video-based soundtracks. Videos are always in MP4. Since there are multiple music styles within the same video, I create different projects for each music style, but always using the same video. The problem is that the more Trim-in time is getting ahead, is slower is the playback. I there any way to stop this slowness when playing back an MP4 with high Trim-In time?