Yes, WindowServer is another one that pushes the CPU for no obvious reason. Browser windows are a candidate.
Thanks everyone for your suggestions. I’ll keep trying some things. It’s not a major problem but probably has some impact and it’s annoying because its a mystery.
Any updates from your end?
I think I have resolved mine, did contact Apple support, did NVRAM, SMC reset, booted in safe mode ... no change, then started to unplug external hard drives, and started with the spinners - and it stopped, I mean Finder CPU usage was normal.
Turned out that my backup drive (I erased the entire drive ~ 1 month ago and tarted TM backup from scratch) did not let me run FirstAid, it returned an error code ... erased the drive again and now FirstAid is not reporting issues anymore, but I will keep a close eye on it.
Apple support through it might have been Finder recalculating file size on the drive over and over but that I not positive ...