High CPU in kwin_x11 and Xorg.bin when Screen is Locked

When my screen is locked, kwin_x11 and Xorg.bin are both using a lot of CPU:

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND                                                                                                                                                                                                
   4541 Alex      20   0 3916320 317172 218040 S 69.77 0.972  32:39.67 kwin_x11                                                                                                                                                                                               
   4301 root      20   0  785208 210124 153988 R 52.16 0.644  26:34.57 Xorg.bin                                                                                                                                                                                               

This is happening on both my Tumbleweed computers. On my laptop, this started several months ago, and seem to only happen when I have external monitors connected. The other computer was just installed yesterday (and also has two monitors connected.)

What can I do to find out why (and stop it)?