Why after 40 years of PC, every OS is still hanging/slow and I can’t understand why?

It happens on Mac, Windows, Linux, from time to time some programs get very slow and no matter how I try to anlyize it, I never understand the "reason".

Restarting almost always works for sometime. But no matter my 16GB, it will happens again.

I never got to understand why a single program, on my inactive PC is slow showing files for example, opening windows, refreshing etc. never.