No it won't and it never will. It's a very simple equation:performance in Xmir should end up being almost identical to performance with just X
DE > Xorg > Hardware
DE > Xorg > Xmir > Hardware
There's no way you'll ever get less by stacking in more. Just not going to happen. All the composition still has to happen in Xorg, all the faults of X will still be there. Now it'd be a different issue if you were running a native DE on Mir, and then running legacy apps on top of Xmir. But running a whole DE under it will never give you good performance.