kernel_task up to 450% CPU

Your Mac is overheating. The kerne_task will put the CPU chip into a low power idle mode so that it can cool off. But because the scheduler sees that kernel_task is holding onto the CPU for extended periods of time, attributes high CPU percentages to kernel_task.

Quit some processes that maybe running in the background. This can include browser tabs that have mis-behaving Javascript.

Depending on your model Mac, if you have external monitors, the integrated GPU (part of the CPU chip) will be working harder to drive lots of pixels. Try disconnecting the external monitor(s).

If your Mac lives in a smoky or dusty (OR BOTH) environment, the internal cooling fins can collect dust that gets glued on by smoke particles and makes it difficult for the Mac to cool itself.

If this happens a lot, consider getting a laptop pad with built-in cooling fans.

That can be anything from a software bug to data corruptions to failing hardware to overheating hardware.

Yes, in some macOS versions, Apple reportedly used kernel_task to block cores to assist with cooling, as counterintuitive as that is.

You’re going to have to monitor and see what provokes kernel_task activity.

Some combination of hardware and installed software and settings and connections is causing this, complicated by the possibility of latent errors arising from storage or memory or graphics or elsewhere.

Some folks have found they can summon excessive kernel_task activity with an external display monitor with a particular hardware connection and resolution, for instance.

Console logs might provide some clues around excessive activity, unfortunately those same logs can also provide misdirections.