I need to see your CORE temps
where it says CPU temps, that is measuring your socket temperature. See if you can turn on Core temp measuring on HWmonitor. though, your socket temps seem pretty low. Almost too low. how long did you run prime for? Also your voltages are REALLY drooping. When you put your CPU under load, it is throttling back the supply of power to it for one reason or another. This is what is causing your low clock speed. what you need to do is go into your Bios, and enable LLC (load line calibration) it has several settings but i would put it at medium or high). Also, in your bios, Disable ALL power saving features that you may have enabled. the power saving features throttle back your voltages, which could also be causing your low clock speed. I honestly think this is what is causing your problems. Your temps seem fine because the socket temp is usually hotter than your CORE temps, and if you are in the 35-40c range on your socket, your cores should be fine.
After you enable LLC, and disable the power saving features, run PRIME again and watch your CPU voltages. they should stay pretty static and not fluctuate too much.
Hopefully this solves your problem. If you need anything else just pm me!