If you have installed Ubuntu on your old computer or the low-end netbook, you will know that the Unity desktop manager that comes shipped with Ubuntu is very sluggish and non-responsive at times. This is because the Unity desktop requires powerful hardware to run well. A good alternative is to install the classic Gnome desktop. It is not as resource-intensive as Unity and will work just fine with any older hardware.

Note: The “Gnome classic shell” is now known as “Gnome Flashback.” The user interface remain the same.

Full article here:
How to Install Gnome Classic Shell in Ubuntu (Make Tech Easier)