with Pseudoregalia's PS1/N64-inspired visual style, it's easy to think that it'd run quite well on lower-end hardware, but in reality, the game is pretty heavy on the GPU - especially if retro resolution scaling is disabled. as far as I can tell this is a symptom of the game being built in Unreal Engine, which can be pretty straining on older hardware.

however! there are some graphics settings you can adjust to disable or reduce some visual effects and improve performance. the game's visual style helps a lot here, as most if not all of these settings have next to no impact on the game's visuals. but these settings are not exposed in the game's own settings menu, so we'll need to do a bit of digging to find them.

the settings we're looking for are found within the game's configuration data, which can be found here on Windows: not sure how to get here on the Steam Deck someone told me!
there's a bunch of files in this folder, but most of them aren't important here. the settings we're looking for are in the file GameUserSettings.ini - open that up in your favorite text editor.
at the top of the file you should see something like this:
these are the settings we'll be messing with in this guide (except for sg.ResolutionQuality - if you wanna change the game's resolution, it's best to do it in-game, as this value just reset itself when I tried to change it). you probably shouldn't mess with the other stuff in the file, as all the other potentially useful settings I found in there you could just adjust in game.

each line in this section refers to a graphics setting we can change. from what I can tell the values range from 1-3 - they're all set to 3 by default, which contributes to the game's heavy GPU usage. all we need to do to boost our performance is to replace all the 3s here with 1s, like so:
...though, depending on how powerful your hardware is, you don't have to reduce all of the values. on my laptop with a dGPU, I was able to get a significant performance boost by setting just sg.PostProcessQuality and sg.GlobalIlluminationQuality to 1. if you're unsure, I'd recommend starting with just those two, checking performance, and lowering the other settings as you see fit. or you can just set everything to 1, if you don't feel like messing with that.

once you've made your changes, save and close the file.

here's a quick comparison of my GPU usage and FPS before and after editing the .ini:
(if you can't tell, there's a split in the middle; left is original, right is with the new config)

with everything set to the lowest value, anti-aliasing on, and retro resolution scaling disabled, I was able to get 85ish more FPS in the Tower Remains without any drop in quality (as far as I can tell, anyway). obviously though this area is pretty empty, so I did some light testing and jumping around in a couple other areas. with the new configuration, the lowest drop I saw (not counting area transitions) was 90fps - which fell to 30fps when using the original config file in the same area.

so obviously you're not supposed to do this. the game was made with the default values in mind, and we're not able to change them without surfing into the game's internal files. so you should probably expect some visual artifacts.

in my very minimal testing, I only noticed the walls flickering/becoming see-through while falling down the final boss entrance. setting sg.ViewDistanceQuality back to 3 fixed it for me.

but yeah, depending on what settings you change, some effects might be disabled, or some visuals might not look quite right, but it shouldn't be all that major. if you're having visual issues, try setting some of the settings in the .ini file back to 3.

if you've been having performance issues with this game, I hope this guide helped - it's my first one so I hope it was easy to follow :)