Something came up that I have not noticed before - when I select any kind of upscaling when playing docked to a 4k TV, even old and 2d titles will start to stuggle. If I play at 60 frames with linear, Integer scaling would push me down to 40 frames, FSR is around 30 frames and NIS is around 15-20 frames, and this is true for almost any game. What is worse, is that I have better frames running native 4k than having upscaling in older titles. I remember not having such issues before so maybe anyone could advice?
So your Deck will display your screen at whatever resolution, so if you’re on a 4k TV, it should keep the resolution at 4k.
Now in the Steam properties of the game itself, is a Resolution selection. This affects what resolution the app/game is launched/rendered at. This doesn’t necessarily change your display resolution. When the rendered resolution is lower than the display resolution, SteamOS will do up-scaling, using the method selected in the quick access menu (FSR etc).
All that to say that you should make sure you’re using Default or select 1280x720 in order to get decent performance on external displays. If you have Native or a higher resolution set in the Steam Properties, this could be what is resulting in the system struggling with the FPS.
In the Quick Access Menu itself, FSR is usually a pretty good option for the Scaling Method. This will use AMD FSR’s scaling algorithm to do the up-scaling. An FSR Sharpness setting of 2 or 3 is usually good for counteracting the smoothing effect on the external display due to the upscale.