Why is kernel_task using some much CPU from my Mac?

If the kernel_task is using 100’s percent CPU, then your Mac is over heating, and the kernel_task is putting the CPU chip into a low power idle mode so that the chip will not melt inside (literally melt).

This is cause by either processes constantly demanding CPU, memory, storage access and/or network access.

Look at

Applications -> Utilities -> Activity Monitor -> View (menu) -> All Processes -> CPU (tab)

and see if there are obvious CPU hogs.

You could have a failed fan, and there is no enough airflow to cool the CPU.

But most likely is Dust/Pollen/Smoke Particles clogging the air cooling channels. Suggest having the inside of your Mac cleaned.

kernel_task runs everything, so we need more info...

Open Activity Monitor in Utilities, View>All Processes, sort on CPU%, see if anything using too much CPU%, click on Memory tab, what is eating up the most RAM?

Or for more in depth results...

This will not display any personal info...

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...