> C is just "high level assembly" for systems programming

It is not, and that's the issue. If it was just "high level assembly" there would be no UB, and no need to have UB. Instead you have UB (although arguably not all of it is really needed) because you need optimizations, which in turn you need because otherwise C would be too slow for that "operating system" job.

> code which is called thousands of times per second

Scripting languages can easily have loops running thousand of times per second and even more. You're off by some order of magnitudes here if you want to describe operations that happen in operating systems.