The game seems to default the resolution to something way higher than the Steam Deck's resolution with no direct way to set it manually other than adjusting the resolution scaling. This causes a significant performance hit, leading to sub-24 fps even on low settings. I was able to fix this by setting the resolution scale to 50% (there was no noticeable visual difference), and the performance improved drastically. I'm able to play on medium settings and get 40-50 fps with minimal drops.