I'd say that the majority of the time it maintains at least a 480p image, by eye- although it can dip depending on what is going on in the scene. It might even go higher.. I'd say that overall, it is better to play it while docked- it maintains a 720 resolution, with strong antialiasing; you're going to get the best results like that. Either way I think the game looks fantastic, and is a lot of fun. The oled screen makes the colors more distinguished- I think XBC2 is very pretty on the Switch Oled.
