Heiko Tietze wrote:I never had issues with repainting.

Have to observe it for some more days but I want to blame the export __GL_YIELD="USLEEP" for daily crash of kwin. System hangs for a few seconds and then restarts display effects or whatever (don't remember the exact error message) and runs smoothly after this "crash". However, it's quit annoying to have this hiccup every day at an unpredictable time, so I commented out the setting for now.

It will less be the yielding strategy (if "usleep()" crashes for you, you've other troubles , but rather the implications.
On a doublebuffering (not triple buffering, check Xorg.0.log), swapcontrol (v'sync) will be disabled and glSwapBuffers return immediately (and because of that, KWin will use a different strategy, because we don't have to care about loosing time in the glSwapBuffers call)