After the update to Plasma 5.25 beta, if I run "kwin_x11 --replace" in the terminal, the KWin instance that launches make windows redraw and move what feels like one frame per second. The cursor moves smoothly. The resource usage stays normal. Running "killall kwin_x11" makes windows draw fast as expected. Only a reboot gets me back to a normal KWin, though.

The only time I've seen this symptom before was due to the 3rd-party Parachute effect:
https://bugs.kde.org/show_bug.cgi?id=422625
— but I haven't used it since.

STEPS TO REPRODUCE
1. Open Konsole
2. kwin_x11 --replace &

OBSERVED RESULT
A slideshow

EXPECTED RESULT
KWin running normally

SOFTWARE/OS VERSIONS
Operating System: openSUSE Leap 15.3
KDE Plasma Version: 5.24.90
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.2
Kernel Version: 5.3.18-150300.59.68-default (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i5-10210U CPU @ 1.60GHz
Memory: 15.3 ГиБ of RAM
Graphics Processor: Mesa DRI Intel® UHD Graphics