You just have to write a tiny part that uses a lot of CPU in C++/asm or anything else.
Much of code's performance isn't really reflected on to the scalability since mostly a tiny part of code is really ran a lot of time, and the other parts are just glues or management stuff or rarely used(not used in scale) features.
It depends. You aren't going to make a very fast modern codec encoder or decoder using Python. The hotspot ends up being the vast majority of the process. That management/glue layer becomes very thin, amounting only to feeding in the bitstream and reading back the raw video frames.
Much of code's performance isn't really reflected on to the scalability since mostly a tiny part of code is really ran a lot of time, and the other parts are just glues or management stuff or rarely used(not used in scale) features.