If you’re not maxing out your CPU doing one file at a time, you can use parallel to run concurrent jobs. Make sure you have GNU parallel installed, then just put parallel in front of the command in the for loop: parallel ffmpeg ...
In this specific case: file holds name.mp4 the % part tells bash that something needs to be stripped from the end. The .* tells bash that everything after the . (dot) needs to be removed. You now need to add the mov extension to the string that is left.
EDIT: This isn’t greedy btw. A file named x.y.z.mp4 will be renamed to x.y.z.mov. If you do need to be greedy use %%.* x.y.z.mp4 will now end up as x.mov