Ubuntu not defaulting to Wayland is about features, not performance or efficiency.
In theory, once enough bugs are solved, Wayland will be the more efficient option because in a Wayland session there is only one display server process. In a Xorg session there are effectively two display server processes and there will always be a roundtrip penalty for some things between the gnome-shell and Xorg processes. But while using legacy apps that only support Xorg (like Chrome/Chromium) you also have the two process problem in Wayland, as it has to run Xwayland.
In practice, Wayland is feeling faster to me right now. But I’m suspicious as to whether there are any bugs in Xorg sessions making those slower, or if we’ve actually started to reach the endgame for Wayland over Xorg.
Also keep in mind that a display server maintaining a lower frame rate will sometimes be more power efficient, even though that’s a bug. But we don’t aim for power efficiency as the primary goal. The primary goal is to maintain maximum frame rate. So yes maybe the smoother performing option will use more power.