It even happens with power efficient devices. All Macbooks will run at their lowest clock speed with a dead/low battery (even my M1), My Thinkpad T14 with an ULV CPU and it’s odd. It tries to limit total system power to around 25 watts, even though I have a 100 watt power supply connected. My theory is that since 30 watts is the lowest power supply it will run off of it’s trying to keep that 5 watt buffer. Unfortunately that means my CPU runs at 800mhz doing anything but idling. Laptops with dGPUs often just wont work at all, or are so far limited they’re unusable.

Some older laptops like my Thinkpad X220 will run at 800mhz on a 65 watt charger, but on a 90 watt charger it will run at full speed. But unfortunately in the days of USB C that makes things a lot more difficult.