I know the GoPro’s MP4 video is not ideal for FCP, (and I know that more RAM would definitely speed things up) but I’ve used it (the exact same setup) for two previous dance recitals, and it’s never caused problems like this. In this case, the camera was on the back wall of the stage and while I’ve got 1.5 hours of MP4 video from it, I’m only using a couple minutes of it, so thought I would only render those bits, rather than the whole thing. Because the rendered bits play back fine …
I can understand if it would cause the spinning beach ball to show for a few seconds/minutes, but to have to restart the whole COMPUTER each time??? Sounds like something else if happening under the hood …