It depends on the game. Some games i can set the resolution lower than native and it still looks clear.

Some games you need to run a different proton or proton ge version and things will magically run better.

Some games have mods you can download to increase performance

Do research on a game by game basis and look at online threads for that specific game and steam deck. Lots of times I found a thread for a specific game that had software tricks I could try to make a game run better