Computer keeps freezing after installing Windows 10. What to do?

Love everyone

I'm currently making a laptop for a colleague. It usually works very well, but this time I have problems that I can't fix on my own.

Yesterday I installed Windows 10 (via USB stick) and previously formatted and repartitioned everything, so really thoroughly cleaned. Before that, Windows 8.1 was on the computer.

Since I have installed Windows 10, the laptop has jerked violently. The mouse pointer stops approx. Once every second and moves (regardless of whether touchpad or mouse). The windows also take longer to open. In general, the laptop was very slow before, but this time I'm sure it's something else. What can I do? I have tried various things from the Internet and nothing has helped.

Computer model: HP 14
Processor: AMD E1-2100 APU, 2 core
Graphics: Radeon (TM) HD Graphics (8210) 1.00 GHz
RAM: 4.00 GB (3.47 usable)
System type: 64-bit
Windows 10 Home, Build 18363

It is brand new, so nothing on it that I installed manually.

CPU not overloaded, but fluctuates every second (always goes up and down steeply)
Superfetch is already deactivated
I've already optimized it for performance
I've already done Windows Shell (scanhealth, checkhealth, restorehealth)
According to Windows, drivers are all current
everywhere else I'm shown that everything is OK

Ok, but would you prefer Windows 7 instead of Windows 8.1? I always found 8.1 terrible.

There's just no more support regarding the program and virus patches of the system itself!

How can I downgrade this?

Since WINDOWS 7 is no longer "supported" by Microsoft itself, possibly here:
https://www.chip.de/...08928.html