Make sure to put your TV in “gaming mode” or whatever it’s called in yours. Otherwise it’ll do some extra calculations to improve image quality which will cause noticable input lag. I’ve struggled with that issue for a while, thinking the Deck itself caused the input lag when displaying on an external display, but it actually was the TV which caused it. After setting that mode, together with FSR, it was basically the same performance as in handheld mode from what I can tell so far.