Mirroring the main screen causes measurable reduced performance in Wayland. The issue is not reproducible in X11. STEPS TO REPRODUCE 1. Plug a second screen output 2. Mirror screen output by replicating screen in the display configuration OBSERVED RESULT Reduce performance in a native game benchmark : Shadow of the Tomb Raider Around 4% EXPECTED RESULT No impact on performance. In X11, there is no performance impact at all. SOFTWARE/OS VERSIONS Arch Linux/KDE Plasma: KDE Plasma Version: 6.3 KDE Frameworks Version: 6.10 Qt Version: 6.8.2 Full example here: https://youtu.be/cBR7ogYafpg
This is when direct scanout is not working, right? Iow, when the "show compositing" effect does not go away in fullscreen?
Sorry for the late answer, I thought I would get a notification if someone answered. I just redid the test to make sure. Composting is not showing on screen and there is a difference of 3% / 4% on the same game bench. This is not reproducible on X11, the bench is the same weather I duplicate the screen or not. Let me know if you need more info.
This might be caused by rendering of the two screens not being synchronized - so sometimes KWin uses the app's buffers for longer than without mirroring. Synchronizing rendering for mirrored screens is planned, so let's wait for that to be implemented and then check again.