Windows, by default, acts differently when on battery compared to plugged in.
On battery, performance is lowered and integrated graphics is used.
Plugged in, you use discrete graphics and max cpu performance.
Unfortunately, that generates heat that some light coolers are not well equipped to handle.
As a result, you may be thermal throttling.
Run an app such as HWMonitor or HWinfo, you will get the current, minimum, and maximum cpu temperatures.
For intel processors, if you see a max of 100c. it means you have throttled.
The cpu will lower it's multiplier and power draw to protect itself
until the situation reverses.
At a lower multiplier, your cpu usage may well be at 100%
What can you do?
First, see that your cooler airways are clear and that the cooler fan is spinning.
It is counter-intuitive, but, try changing the windows power profile advanced functions to a max of 90% instead of the default of 100%
You may not notice the reduced cpu performance.