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Nothing happened.
I've seen some Notebooks that required restarting when changing power settings for the charging part when the CPU speed was involved.
It is power saving, or could be thermal management, it is meant to do that in certain modes.
Battery icon bottom right and select Best performance rather than best battery life.
On mine different CPU Best performance runs 2.5 GHz to 3.1 GHz, on Best battery life 0.8 GHz to 3.1 GHz.
It does that on the fly with no restart on mine.
Also check in the Power Options there is only one in there now, Balanced, though you can create another if you wish.
Balanced > Change Plan settings > Change advanced power settings > Processor power management.
Max processor state 100%.
Min processor state nominally 5%, however the actual processor clock will have a certain range depending on the particular CPU so it won't go that low.
I would not put that at 100% as that will cause the CPU to use more power unnecessarily. Just check that it does actually go higher when demands are made of it.
Manufacturers often put in their own section in here as well just to complicate it.
It maybe thermal management when charging as that that would raise the internal temperature, thus clocking down the CPU.
I know this is a week old, but check the jack on your power adapter for the "center" pin. If that's missing or broken in the wire, the PC will think it's an 'unknown adapter' and lower the cpu accordingly.
Basically, it's either your adapter or the jack on your machine.