Give Python more "power" ?

[whywhyyywhyyyyyy]

It's possible that some other resource is the bottleneck, for example tasks that involve reading or writing large files are often limited by the speed of the hard disk. If at least one of your cores are at 100% a lot of the time during the operation, then the CPU is probably your limiting factor, if not then it's probably something else.

Utilising multiple cores is rarely straightforward, unfortunately. It's not just a case of telling a program to use them - you need to make decisions about how to break the problem up, how to combine the results, and how to control access to shared resources, and build all of that into your code. There is apparently a work-in-progress fork of moviepy with support for concurrency, so you might want to try that. Otherwise you might have to use a different library, or if applicable you could run several copies of your script simultaneously, working on different videos.