Tell Windows I'm busy, stop using so much CPU in background

Hi, it would just be a guessing game as to what might be causing CPU usage peaks - presumably even when your PC was nominally idle.

There are I suppose two possible sources:
- routine schedules intrinsic to Windows
- usage arising from programs you have installed.

It would be helpful to know the origin: Process Explorer (free from MS) can give more information than task manager.

To consider:
a. Background scheduled events e.g.
Change Optimize Drives Schedule Settings in Windows 10
Enable or Disable Scheduled System Maintenance in Windows
Schedule Scan in Windows Defender Antivirus in Windows 10
Change Optimize Drives Schedule Settings in Windows 10

b. Scheduled events due to 3rd party programs e.g. AV scan - but could be anything.
For example, you can set up scheduled registry backups with a 3rd party program.

c. Updates- could be
- automatic Windows updates
- automatic installed program updates (traditional desktop progs)
- automatic Store program updates (these are relatively frequent

You can check the list of scheduled tasks conveniently using Nirsoft's Taskschedulerview- this provides a linear list of tasks, easier to view.

Try setting process priority:
How to Change Process Priority in Windows 10

Process priority should help, but you need to discover why your idle CPU usage is so high.

Compare what it is with that after a clean boot.