I very rarely use big picture mode. I’m mostly on a KDE desktop. I’ve set up a shortcut to open Steam through gamescope in Big Picture mode for the rare occasion that I need it. In that case KDE’s wayland session keeps running in the background.

I have also set up gamescope with Steam as a separate login session but I can’t remember if I ever felt the need to use that.

Usually I just have Steam running in desktop mode in the background for the controller settings and the mostly superior on screen keyboard. I never noticed any slowdowns in games. I even managed to get Cities Skylines to run more stable than on SteamOS. But that might be due to zram.