Notebook Operating System and Recovery

Hello.

I have a soon three year old HP Laptop "14-bp081no".
My wife and sister have the same laptop, 2,5 years old.
Now I have a problem, which they do not have. I've had this problem since December 2019. Mind that I have installed all the Windows updates, HP updates, and reinstalled the PC.

The problem:
When unplugged, or plugged but charging, the PC is extremely slow. Loading a web page takes time. Opening up the file explorer takes time. Each mouse click returns a delayed response. The CPU, memory and disk usage are low, 10%, while this problem occurs. Even when I'm playing a song on YouTube, I get a distorted sound due to lag. But as soon as the PC is charged minimum 80% charged put plugged, everything is good. The battery settings are at maximum while plugged and unplugged. I reinstalled the PC in spring, and it's still like new, there's no installed programs or files on it, except Google Chrome.

I have always used if for browsing and writing documents. Some YouTube of course.

I did some CFD simulations in fall, which requires high usages of CPU for several hours, no idea if this can harm some components, that would have just been strange. I scanned the PC for hardware issues with the "HP PC Hardware Diagnostics Windows" and "Windows Memory Diagnostics", but the results are all good.

I long struggled with trying this and that. Eventually I isolated the problem to occur while unplugged or charging. Everything is good while it's plugged with minimum 80 % battery.

What should I try next? BIOS update? I have no idea. Everything is updated according to Windows and HP.

Thank you.