If you don't know what you're doing and have the right tools, you will break your shit.
And no, I won't tell you what they are, because if you can't figure it out it's a really bad idea (and also because I have no fucking clue, but no one need to know that part).
It does run on steam deck (though at lower settings with FSR to get 40 mostly stable).
Open world games with some complexity generally take a decent amount of power. You have to load a good number of surrounding objects at any given time, with a pretty wide view on the zoomed out view. There are also other characters/animals doing stuff, environmental effects, and a healthy dose of passive checks on the environment against various traits of your party to see if your character identifies any of the secrets all over the world.