14 DAY TRIAL // Ubuntu developers have decided to replace the default NVIDIA proprietary drivers with the Nouveau version, in one of the daily updates for Ubuntu 14.04.
Ubuntu 14.04 is still under heavy development, but if I remember correctly, Ubuntu 13.10 had the Nouveau drivers enabled by default when the user first installed the distribution. This might be the case with Ubuntu 14.04 LTS when it ships.
If Ubuntu developers decide to include, besides the default Unity7 session, the new Unity8 and Mir session, users will need to have access to the Nouveau drivers, which provide support for Xmir.
In any case, it’s interesting to note that I immediately noticed something was wrong after a major update. I hit dist-upgrade without looking, restarted, and everything started to get sluggish. I’m not trying to undermine the effort of the Nouveau developers, but it’s not a good sign for the open source drivers.
So, if you are running Ubuntu 14.04 already and you notice a drop in performance, be sure to check what drivers you have installed.
Update: I've been told that Unity8 will not be included by default in the installation image, but it will be available in the repositories.
Also, switching from NVIDIA proprietary to Nouveau drivers might be just a bug common in the development process. Ubuntu 14.04 is still in the Alpha stages, so this is more than likely.