Laptop performance sporadic following upgrade. reason unclear

Running a Compaq Laptop with W10 and I'm experiencing something new.
First the details of the hardware:
Winver: 10home. 20H2 19042.1052. Feature Pack 120.2212.2020.0
PC is an AMD E1-2100 APU with radeon HD (1gb), 4gb of DDR3 installed, on a 64bit system.

Following "issues" it's had a blank-drive reinstall from an existing disc. The 21H1 build refuses to work on this machine.

Initially the install appears flawless, but there is something hinky going on in the background.

There is something interfering with the smooth running of the system, it's using up about 2 out of every 10 cycles is the only way I can describe it.
Everything works, but there is a noticeable interrupt that takes up 1 second out of every 5. mouse moves 4 inches on screen and stops for a moment, then continues. If there is a process, like the update screen with it's spinny blue dots, that "pauses" every few seconds.
Task manager has the CPU running at around 50% pretty much all the time, never less than 35% and sometimes at 75%. it's like there is a hidden process that's chewing up the available runtime. all TM says is "system", there's nothing strange in the neighbourhood, this is a clean system.
Tried reformatting the HDD again and re-doing the install, same result.
Reinstalled offline, refused internet connection and all was good for about 4 minutes when it burped a random beep and all was stutter-y again.
Tried it with and without updates, nothing makes it different so far that I’ve tried.
Ran in safe mode and all is ok, so it's possibly a system process, but I can't identify it.
I have ordered a bigger stick of RAM to give the CPU some help, but it's just plain annoying that it's not behaving. I could understand slow all the time, or BSODs, but can't see what it's doing this.
Is it simply that this laptop can't handle Windows10? Surely not?

Any help is greatly appreciated

Thanks for the reply, it shipped with 64bit os from factory, sadly that was Win8.
I didn't get my hands on it until after it suffered the HDD fail, so don't know the W10 os versions, but apparently it was ok.

Safe mode sees it running ok, no stuttering or anything, nice and smooooooth.

There seems to be a degree of compaq support for W10 systems, including 64b drivers for <stuff>

I've not used process explorer yet as there's nothgin looking suspicious and I'm getting racked off with the stuttering. it's like the fuel filter is blocked and it won't rev beyond an idle without stalling.

ok, so the process explorer finally installed




anti-malware exe seems to be memory hungry, but not CPU-obsessed.

Perhaps more DDR3 will help, there's some on the way, but the process interruption is such a PITA, I want to fix it.
Do I just leave the antimalware to run and see if it clears up in a while?

What type of hard disk are you installing W10 on? SSD or HDD.

it's going on an HDD style, don't want to spring for a SSD yet. This is supposed to be a homework/SOHO machine, not a gaming rig.

Basic spec, nothing fancy and a clean formatted install, should be nothing to clash

this is why I was unsure of the use of the process explorer, other than the antimalware executable which is gobbling memory like a fat kid eats icecream, there is no reason for the system to be stuttering.
I would like to try a W10 32b install, but can't find a version to build a disc.
Failing that, I'm going to shove Ubuntu on there and be done with the whole thing.

"Computers are like Air Conditioning: Open windows and it all stops working"

- - - Updated - - -

also: msconfig and shut down all services at boot prevents me from being able to log back in again. Unable to access the password screen, or the pin screen or anything really. Probably because cryptographics are stopped.