Comment 9 for bug 1908082

@Tynach, the symptoms you describe do advocate in favour of the toolchain/optimizations hypothesis. If upstream builds are not affected, wherease Ubuntu builds are, when both are run on the same platform, surely there is a difference in the optimizations used to produce the binary under test.

That said I've tested your sample shader in shadertoy on Ubuntu 21.04, and while the framerate is not super constant it does hit 60FPS regularly (chromium is behaving better, also around 60FPS but with a much more constant framerate than firefox). That's with the firefox Ubuntu build, and the chromium snap.