SUMMARY
After a variable amount of time, kwin_wayland starts consuming a high amount of CPU. Typical CPU usage of kwin_wayland while system is idle is 0-2% on this machine. When the problem appears, consumption rises to 8-12% or even more. Framerate is lower and CPU fan accelerates.
This only started to happen when I upgraded to Plasma 6. It never happened with Plasma 5.

STEPS TO REPRODUCE
After trying many things, I found a weird way to reproduce. It does not happen 100% of the times, but doesn't take much to reproduce here:

1. Close every application/window.
2. Open VLC and play a video (in Manjaro VLC 3 is using XWayland by default);
3. Seek a few times into the video or/and try to resize the window a bit. This makes kwin_wayland consume a bit more CPU, going above 20%, which may be normal;
4. wait a few seconds and close VNC.
5. In some cases, kwin_wayland goes back to 1% or less, which is OK. Go back to step 2 in this case and try again. In other cases, it remains at around 10% CPU or even more.

OBSERVED RESULT
System framerate lowers. It even affects video playback in VLC, making the system pretty bad to use. Fan accelerates, leading to higher battery consumption. The situation does not fix by itself, I need to reboot or logout/login.

EXPECTED RESULT
With the provided steps, I'd expect kwin_wayland to go back to the usual 0-2% CPU usage after VLC is closed.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.8.8-2-MANJARO (64-bit)
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0

ADDITIONAL INFO:
I tried to disable vaapi acceleration. Nothing changes. I tried to blacklist nvidia (I only use the Intel GPU anyway), nothing changed.