Bug 475592

SUMMARY
Vsync issues in KWin Wayland session, especially when running multiple programs at once and/or using multiple monitors. Issue is not KWin specific (appears in other Wayland DEs/WMs such as GNOME 44 and Hyprland) but it does appear to be fixable as the latest version of GNOME (45) has completely fixed this problem on my system.

VRR is not enabled on any displays on this system


STEPS TO REPRODUCE:
This isn't the only way I can reproduce the issue, it's just the most common one I run into.
1. Start a video, such as a web browser video one one monitor. 

2. Start another program, such as a game or eglgears on another.

3. Watch the apparent and measured framerate of the compositor and applications on every monitor stutter or lower, especially with mixed refresh rate displays. This happens regardless of the current load on the GPU.

OBSERVED RESULT
1. After starting a video, for example, the compositor on all monitors appears to begin to have framerate issues. It won't necessarily be the same framerate across all monitors, but the issue is strongest if one monitor has a lower refresh rate than the others and that monitor is running a video or 3d accelerated program. Even if the framerate drop is minor, there is still at the very least noticeable microstuttering.

2. After starting another program on the same monitor or a different one, the frame rate of that program will also noticeably decrease in the same way as the compositor if that program is vsynced.

3. Overall, the result is a very choppy experience on multi monitor setups when multitasking.

4. Sometimes, an application will cause the entire compositor and every application running on it to appear to drop to its framerate completely. For example if a 3d accelerated program runs at 30fps, sometimes everything will be stuck at 30fps including other apps on other monitors until the program is closed.

EXPECTED RESULT
Running a program should not affect vsync of other applications, especially on other monitors. Other operating systems like Windows do not exhibit this behavior on this hardware, and GNOME 45 using the same hardware and on the same system does not either.

SOFTWARE/OS VERSIONS
Linux Version: Arch Linux (Kernel linux-ck 6.5.7-1-ck)
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.11

ADDITIONAL INFORMATION
System Specs:
i7-11800H CPU
Intel TigerLake-H GT1 [UHD Graphics] GPU
NVIDIA GeForce RTX 3060 Laptop GPU

While this system does have an Nvidia GPU, all of my external displays are connected via thunderbolt 3 dock into the iGPU to avoid Intel/Nvidia Wayland issues. There are no displays on this system connected to the Nvidia GPU when I've observed this described behavior. This appears to happen even if the nvidia gpu is disabled as well.

This issue appears to happen with heavily varying intensity unpredictably, but it is always present to some extent. Nothing I've tried so far has been able to remedy it.