In the latest revision to Mir at the time of writing, rev 1102, there is an important performance fix. Surfaces that are fully hidden by other surfaces (windows) are no longer being rendered with this fix. The original issue and bug report was brought up when finding out for Ubuntu Touch that when launching the clock app -- which re-renders the screen every second to show the new time -- would still be drawing in the background even when the application was minimized or covered up by another app.
On mobile devices in particular -- when commonly just showing one application at a time -- this can cause some serious performance overhead and decrease in battery life by continuing to render hidden application surfaces when they aren't even being scanned out to the screen. Now, if the surface is completely hidden, Mir won't bother doing this useless rendering.
Revision 1102 also landed other changes as pointed out by the Launchpad Bazaar history (as of late it seems the developers have been squeezing multiple changes into single commits rather than cleanly separating out the changes in Bzr). The commit just happened on Thursday so will likely just be pushed to Ubuntu Touch as a stable release update.