This has not been my experience with Kubuntu 20.04 or 22.04. I have occasionally (like maybe once every six months?) had an issue that seemed likely to be Plasma-related, and was fixed by a reboot. And there was an issue with Plasma Search where if I let it search "Recent Files", it would hang the whole UI if I did a search that (I guess) was a match for some recent files. (Worked around that by unchecking "Recent files" in the list of things that Plasma Search searches...) So I mean not 100% issue-free, but nothing like what you describe. And certainly compares quite favorably to other Linux DEs I've tried (Gnome, Mate, XFCE, LXQt).