I initially blamed OBS (Open Broadcasting Studio) for the problem. 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 driver.

Unfortunately I am not quite able to capture timelines of the
/sys/kernel/debug/dri/0/amdgpu_pm_info correlated with the frametimes, as very
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 producing
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=fps, GALLIUM_HUD=fps, in-game fps / frametime counters). But main one
is using modified Mesa overlay.