I initially blamed OBS (Open Broadcasting Studio) for the prob=
lem. But I was
able to reproduce the issue even with OBS recording, previewing, grabbing
frame, or even it running.

So I am almost sure it is hardware or power management issue in kernel driv=
er.

Unfortunately I am not quite able to capture timelines of the
/sys/kernel/debug/dri/0/amdgpu_pm_info correlated with the frametimes, as v=
ery
often reading this sysfs file will block all the rendering for few
milliseconds, skewing results (frametime spikes).

However, I can tell that the temperature reported there is constantly below=
 32
deg C, and frequency looks all the time the same, at least for the GPU core.

The voltage and GPU load reported there is all over the place, sometimes
reporting 0% GPU load, despite the game running in windowed mode and produc=
ing
200 FPS, which should be at least ~30% GPU load from my other measurements.

I also confirmed FPS / frametimes issues with 3 other independent methods
(DXVK_HUD=3Dfps, GALLIUM_HUD=3Dfps, in-game fps / frametime counters). But =
main one
is using modified Mesa overlay.