The render issues you are running into aren’t because it’s a 700MB decent res file, it’s because the codec used is more work to decode, THEN it has to start a new encode… that’s per frame.
I can rip a disc to MOV or AVI and it takes a while. I can take that same file, and re-encode it to MP4 or something and it’s pretty fast. Try to encode that again to MPEG2 for DVD and it’s back to being slow. Some a 2-pass processes as well.
The problem with iMovie is that it’s going to take forever to create thumbnails for the original video, and it could encode slow as well. You want to bring me your files and let me find the fasted process for you, and then you could apply that to the others?