
After some tinkering, I can put up with the 20-30 fps performance. All combat is turnbased.
Modded
Game automatically runs with a controller configuration on Steam Deck. There is no option in the menus to change to keyboard/mouse. Hitting spacebar at the main menu will change it.
Previous Owlcat games have run poorly if there are too many save files. I frequently delete extra saves to prevent this from happening.
Game typically runs at 20-30 fps. However, performance was frequently plummeting to around 10 fps for 30-60 seconds at a time. I made two changes:
1.) Installed "Fast Loading Times" mod from nexusmods.
2.) Disabled autosave in the settings.
I have played roughly 6 hours now without the drops to 10 fps. I will update this if I encounter the problem again, but I'm hoping I solved the issue.

Can't hit a stable 30fps
The game is playable on the Deck, and the controller UI -- while not quite as convenient as mouse and keyboard -- works reasonably well with no fiddling required. Unlike some CRPGs the text and general UI is quite readable, both on a TV at a distance as well as on the Deck's small screen. Kudos there. However, performance is quite poor given how graphically primitive the game is. Don't expect to hit a consistent 30fps. The game looks like it could've easily come out in 2015, the environments are very small, and NPCs have no behaviors to speak of, so it's baffling to me why it performs as poorly as something like BG3.

Cannot reliably hit 30, let alone maintain it.
I've tried a variety of settings and fixes including ones listed on this page and the performance is shockingly bad despite being verified and having a Deck specific graphics preset. I don't know what's changed in the three months since the last post on this page but I'd only call it playable by a razor thin margin, it entirely comes down to your ability to deal with sub-30 frame rates and the classic FSR pixel vaseline blur.

To switch to keyoboard and mouse input, you have to call up the Steam keyboard (Steam+X) then press the spacebar of the Steam keyboard. This will pull up a dialog box asking if you'd like to switch to keyboard and mouse input.
Great game, runs well, but it always starts assuming you're going to use a controller or the game pad. Switching to keyboard and mouse input is unintuitive and you can never set it as the default. Hopefully they'll fix this in a future patch.

Buy it, download & install it, start it, have a great time!

Limit to 40 FPS
GOG version installed with Proton. Game runs perfect with Proton 9 and I have no issue at all playing. Companiom's movement with controller had some issues with pathfinding, though, but very minors.

default setting with vsync to "every second frame" and FSR ultra quality and get 40 fps in standalone and 60+ in docked mode while enabling SSR and HBAO (Which come disabled by default)
getting the deck to sleep, hung the device ONCE

Mix of high, medium and low. FSR to quality with TAA disabled. TAA causes extreme blur with FSR enabled. Leave framerate unlocked as limiting FPS significantly increases loading times.
TDP to 11

Battery life is about 2 hours, and only "complaint" is that it chuggs just a bit when on the ship. (just released though, so may be an optimization thing) All in all, works perfectly as is with the defaults!

Very blurry when FSR and TAA enabled
Does not exit cleanly. Must be aborted.

beta instability is expected
agian it's a beta