XFCE is a great desktop environment that is fully supported and available with Ubuntu. Compared to Unity, it is lighter, its desktop and window UI layout is more traditional and is far most customizable.
If you are finding Unity to be slow and want to try something lighter, try Xubuntu! This is XFCE plus customizations to make it familiar to Unity users and a whole bunch of light applications that typically accompany XFCE.
$ sudo apt install xubuntu-desktop
Though it does not ask for it, make sure to restart after this finishes. This way you get a correctly initialized Xubuntu login manager.
After restart, you will be greeted by a Xubuntu login manager. Remember to pick
Xubuntu session from the top dropdown before you login.
After logging into Xubuntu, you will notice the customizations it has over XFCE. For example, your startup applications and notification applications from Unity are carried over to Xubuntu.
In the login manager, there is a
XFCE session too. This is raw XFCE without customizations from the Ubuntu team. If you login to this and login back to Xubuntu, you might find that the icon, window or color settings from XFCE session will persist in your Xubuntu session. Just a warning!
Tried with: XFCE 4.10 and Ubuntu 14.04