Why is system idle process in Windows 10 running at 99%

This is a good thing!

The System Idle Process is, as the name suggests, just a measure of how much free processor time your computer currently has. So, if System Idle Process is taking up 99 percent of your CPU’s time, this means that your CPU is only using one percent of its processing capability to run actual tasks.

https://www.channelpro.co.uk/advice/9901/system-idle-process-high-cpu-dont-worry-its-normal#:~:text=The%20System%20Idle%20Process%20is,capability%20to%20run%20actual%20tasks.

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