When playing games, kwin usually automatically disables compositing (and if not I have a window rule for it). At some point I wondered why my gaming performance was so bad. I was playing at about 15 FPS instead of 60-100FPS. Opening htop revealed that krunner was hogging all my CPU cores at 100% until I killed it.

I cannot reproduce this behavior on demand, but it has happened ever since. Disabling compositing doesn't directly start this behavior in every case, but it happens quite often. On some days I get lucky and cannot even reproduce it when toggling compositing all the time, on other days it starts as soon as compositing is disabled almost reliably. It has never happened when compositing was on, but once krunner has gone wild, it doesn't stop only because I turn it back on (i.e. I have to kill it in any case).

Reproducible: Sometimes