So the portable version (run via ./PrismLauncher-Linux-Qt6-Portable-9.4/PrismLauncher) behaves much better.
The good thing is that Intel is being detected, instead of Mesa and llvmpipe. I feel like the game still used to run better than 20 FPS but not by a large margin, it’s basically playable when executed as above. Screen resolution still seems to be wrong.
What bothers me the most is that I am 99% sure I had run Luanti, installed as an RPM package, before I switched to Silverblue. And the lag was there, just like in Minecraft - Prism Launcher, flatpak.
It also doesn’t really explain why I couldn’t reproduce the issue in live environment - isn’t the flatpak version the same there? I literally had to add flathub so it’s not possible it pulled an outdated version, is it?