Composition provides no value when there's a full-screen application running, for 95% of cases, and you can tell performance is smoother when turned off. I don't know if the full-screen window is composited itself, but even if that wasn't the case having the rest of desktop elements being composited does nothing and decrease performance. So it would be better disabling composition by default when there's a full screen application, as kwin-lowlatency does, while perhaps allowing composition for some full screen windows if they explicitly request so with a window rule.
Hmm, I'm not sure this is a good thing to assume. Turning off compositing can also result in more screen tearing (it does for me) and it also disables all shadows, which makes the task switcher look ugly when you use it to leave a Full screen window. And when leaving fullscreen mode or switching to another window there would be an ugly flash as compositing is re-enabled. Finally, compositing is always enabled on Wayland so doing would only take effect on X11. If this is something you'd like to do for yourself, it can be accomplished with KWin window rules. *** This bug has been marked as a duplicate of bug 431736 ***