crud/garbage

Strange to ask this about Linux (Kubuntu 22.04 with all updates), but: Occasionally my always on machine starts working VERY slowly. No strange CPU or memory use in top. Nothing unusual in logs (maybe I'm not looking in the right places). Re-boot solves the problem. As "proof" that there was something wrong, shut-down takes several minutes!! And the boot then works reasonably fast. I tried re-booting when there was NOT a problem and then the machine shut down in a few seconds.

What am I missing?