I don't have any raw data for performance numbers and that wouldn't matter anyway because they may not be relevant to your set up; if you're concerned about that you should run a test comparing them yourself on your specific environment. I'm speaking in terms of code complexity here, if you want to follow the threaded approach then minimum two threads will do the job (one for the scenegraph, one for the sockets), and an X compositor should potentially be doing this anyway to avoid lag caused by slow rendering. The difference is that the X compositor is just storing a copy of large amounts of state from the X server, whereas the Wayland compositor would store the canonical data and wouldn't need to worry about falling out of sync with the X server.

Also, the way that X does it is overly complicated and is unnecessary to have protection against window manager crashes. A similar type of crash protection could be done with a Wayland implementation and it could be done in a much simpler way than moving the entire window manager out into a separate process. You just need to have another process that can hold the client fds and cache a minimum amount of state needed to resume the clients, it wouldn't need to know as much as the X server does to accomplish that task. Prior art is in the Arcan Wayland bridge, other Wayland implementations have not implemented this but they could eventually: https://arcan-fe.com/2017/12/24/crash-resilient-wayland-comp...