The recently announced Mir is seeing quite a rapid pace of development with multiple commits to the code base almost everyday. You can track the development from the Launchpad page. Multi-threaded compositing is already up though how well it works is yet to be seen.
If things turn out like Unity, we can expect a first release to not do so well. Only time will tell how many versions it will take before Mir is good enough to truly replace X.org and/or Wayland.