Created attachment 185426 [details] top and intel_gpu_top SUMMARY Playing a full screen video with Firefox on a 4K display results in > 30% CPU usage by the kwin_wayland process. STEPS TO REPRODUCE 1. Run a video playback in Firefox full screen on a 4K monitor 2. Check 'top' on a second monitor OBSERVED RESULT 'top' shows constant CPU usage of more that 30% EXPECTED RESULT CPU usage should not be so high SOFTWARE/OS VERSIONS KDE Plasma Version: 6.4.5 KDE Frameworks Version: 6.18.0 Qt Version: 6.9.2 ADDITIONAL INFORMATION - The CPU is Intel i5-1135G7, I'm using integrated graphic (Mesa Intel Iris Xe Graphics). - I asked a friend to test this scenario with SwayFX - he had around 4% CPU usage by his compositor. Although he has a different platform - desktop with a dedicated GPU. - If a youtube video isn't opened in full screen the CPU usage by kwin is around 6.5%. - Attaching screenshot with 'top' and 'intel_gpu_top' which I run on the second screen. - I wasn't been able to make 'All Screens' screenshot with Spectacle while a video was playing - the result seems to be corrupted. Probably off topic, but could be related.
Looks fixed in Arch kwin 6.4.5-4. CPU usage in described scenario is ~6%. Would be great to get another confirmation but seems like the bug report can be closed.
I can confirm this with my AMD GPU too. CPU usage is now nice and low, as expected!
I'm running 6.5.4 on Debian testing and this is not fixed. My setup has 3 screens: 1920x1080, 1680x1050 and 3840x2160 (this one scaled to 200%, but I checked that the scaling does not impact the behavior described below) Running a 720p youtube video fullscreen in the 4K screen makes kwin_wayland to use 25% CPU. Firefox version is 146.0.1 and runs as a wayland application. Playing mpv fullscreen with a h264 1920x1080 25 fps stream makes kwin_wayland to use ~9% when on the 1680x1050 screen and ~15% on the 3840x2160 screen. When mpv is played maximized instead of fullscreen, kwin_wayland uses a bit less CPU. mpv itself uses ~6% CPU. And if I resize the mpv window such that it uses a bit less than the full screen on the 4K panel, kwin_wayland CPU usage decreases to ~7%. For each simultaneous mpv instance the CPU usage of kwin_wayland increases as much. However, on some unknown circunstances, mpv starts using 100% CPU permanently and then kwin_wayland CPU usage deceases significantly. The video itself seems unnaffected, the only way to detect this situation is to look at signs of high CPU usage. If there is more than one instance of mpv, all of them enters this high CPU usage state simultaneously. On that situation, if I close and run mpv again, the new instance uses 100% CPU too. Then close mpv, do other tasks for several minutes, run mpv and the scenario will be again as described on the previous paragraph. My CPU is an Intel 6700K. mpv version info: mpv v0.40.0 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects libplacebo version: v7.351.0 FFmpeg version: 8.0.1-2 FFmpeg library versions: libavcodec 62.11.100 libavdevice 62.1.100 libavfilter 11.4.100 libavformat 62.3.100 libavutil 60.8.100 libswresample 6.1.100 libswscale 9.1.100
BTW, I forgot to mention that I have no discrete GPU, the above is with the processor's iGPU.