Thanks!
I ran some Unigine benchmarks (Heaven, Valley, and Superposition), and my FPSes are in the 60s on max settings (other people report similar performance). The only posted Linux benchmarks for the same GPU over at Phoronix have it doing around 130 (and, when running Heaven via the Phoronix Test Suite, I get around 100).
Could you recommend some GPU stress testers?
In non-Proton games, there are no problems.
This happened on kernel 5.10, on which I’m having other severe issues; maybe it’s related. 
(That said, I’ve been having Proton issues on 5.9 too, with either no Proton games launching until I reboot or wipe and regenerate Steam’s directory or running at a choppy ~10 FPS after a few days of running fine. That actually motivated me to upgrade to 5.10, where I haven’t had that yet – only the green checkerboard. 
)