I get a solid 45-50 fps in most areas and I set most things to high. Only a few select things like fog and shadow quality I set to low. It ran fine even out of the box. I would recommend using AMD FSR 2 in quality mode (from the in-game menu, not the SteamDeck internal FSR) for a somewhat more stable experience. Mind you I am pretty early on in the game (level 15), so maybe late-game performance is worse. You might also want to check out SteamDeck HQ's recommended settings: https://steamdeckhq.com/game-reviews/diablo-4/