https://www.pcgamingwiki.com/wiki/Pathologic_2

Follow the legendary PC Gaming Wiki tip and add these lines to boot.config located in the installation directory to get a slight FPS boost and reduce stutter:

gfx-enable-native-gfx-jobs=1

gfx-enable-gfx-jobs=1

Drop your graphics settings to Medium in game (it looks fine, but sometimes textures will take a couple of seconds to pop in), set the FPS limit to 30 in Performance overlay to prevent your frames wildly jumping from 60 to 45 to 30 all the time (you will notice it and it will ruin your perception of performance, so just cap the frame rate, no need to use in-game vsync - there will be no tearing and also no choppy framerate).

Runs OK with a stable 30 FPS 95% of the time, playable from start to finish without any crashes or freezes, but be wary that after playing for a long time load times may increase and some indoor sequences may chug frames - just try to make it to a save point and restart the game if that happens to keep your progress.

This is a great game and I did not expect to enjoy it this much on the Deck. Gamepad controls are a bit clunky but it never gets too life-threatening, you don't lose that much speed compared to keyboard and mouse in intense situations.