I chose core 3 at random, core 0 should have been fine as well. When I initially launched the game without any extra launch options, it performed unplayably poorly at the main menu.

I followed other reports on ProtonDB that recommended PROTON_NO_ESYNC=1 PROTON_NO_FSYNC=1, and applying those improved performance to where it was playable initially but degraded during a game into single-digit framerates.

Limiting the game to a single CPU core with taskset solved this problem completely, and the game runs perfectly now. I will try it without PROTON_NO_ESYNC=1 PROTON_NO_FSYNC=1 because they may not be necessary now that it's limited to one core.