Attempting to use any proton versions newer than 5.13-6 in gaming mode will just show a black screen, though you can hear the game's audio. Didn't try desktop mode, but since the performance overlay doesn't register any frames arriving in that state, it's unlikely to work there either. Given the low popularity of this game, some of the load stutter may have been shader compilation. Overall a very positive experience with the right proton version though.