lightweight window manager/desktop environment similar to Unity
Solution 1:
Xubuntu
You may also want to have a look at Xubuntu that comes with the lightweight Xfce desktop. This also includes a customizable launcher (here shown at the bottom) that can also be moved to the left side like in Unity:
Solution 2:
Go for cinnamon !! I am currently using that and it works smooth and fast !!
Main Page - Linux Mint
It has all the good things from Gnome 3 plus thumbs up for performance !!