When rendering 60fps on a 60Hz display every frame takes approximately 16ms to render. Then you have to add TV latency that’a probably around 20ms unless you have a very nice OLED TV. Wireless controller latency is around 8ms I think? Then your imperfect human brain adds even more latency especially when you’re tired after work. That 3ms is not perceivable. Make that 5ms even. Nobody would be able to tell a difference in a blind test.
What you are reading from the readme notes that it calls into xwayland only when gamescope (wayland compositor) is nested within another compositor (say kwin or mutter).
gamescope itself is wayland only, and when run on SteamOS is has no xwayland latency...
It doesn't yet mean that it suffers from the latency measured in the article, as the problem could very well be in something else, such as how KWin integrates with XWayland or how GPU drivers interact with it (especially that Nvidia drivers have a history of making XWayland suffer).