Since a few days / latest update, my cpu spikes to a constant 70%+ usage, mainly by plasmashell. I cannot really find any solution other than killing plasmashell… but yeah… without plasmashell life sucks.
Any ideas on how to go about finding the real issue and solving it?
M
Welcome to the forum!
In order to troubleshoot this, does the CPU spike still happen if you disable the compositor? You can quickly verify that by pressing Alt+Shift+F12 ─ you can re-enable it again with the same shortcut.
If the problem does indeed go away with the compositor disabled, then this would suggest a problem with either the mesa compatibility ─ i.e. OpenGL ─ of your graphics adapter or with the graphics driver itself.
In the latter case, if you have an Intel or AMD GPU, then the driver is in the kernel, and then you could try another kernel ─ I recommend one of the LTS kernel releases such as 5.4, 5.10 or 5.15 ─ but if you have an Nvidia GPU and you use the proprietary driver, then that’s a whole other story. 
You could also add some system info: