[Fazel94]

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.

[kitsuac]

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.