Video settings are saved to the cloud, so the game window may only be partially visible when playing on the Deck if the previous display had a higher resolution.

To fix this: Open the game's Steam properties > General tab and set Game Resolution to whatever the resolution is of the previous display the game was played on. Then make sure "set resolution for internal and external display" is enabled. Then restart the game.

There is a major lag spike every 4 seconds while in-game (doesn't happen in the main menu).