My point is that the clock speed is also dropped for other processes. So it's not a simple heuristic "oh if we have more data than X we can use AVX512 instead o

My point is that the clock speed is also dropped for other processes. So it's not a simple heuristic "oh if we have more data than X we can use AVX512 instead of AVX256", because you do not know what other processes are doing, and they will be slowed down as well. This type of heuristic only works if you are the sole users of the server, e.g. a database server.