how do you verify CPU frequency?
BTW: on RPi5, Raspberry developer still working on the whole stuff around voltage and CPU frequency. There are a couple of issues still, we suffer from.
We have a user who did quite some testing Guide to Pi 5 Under/Overclocking
Or have a look to our GitHub Image | Raspberry Pi 5: Testing and firmware migration script · Issue #6676 · MichaIng/DietPi · GitHub