For some people, Unity environment is difficult to grasp at first due to its nature. For others, Unity performance is very slow due to its shadows, animations etc. Well, if you want to replace Unity and go back to good ol’ Gnome in Ubuntu 14.04 and install Gnome Flashback, then follow below steps. This will install Gnome Flashback which is a lightweight desktop UI and won’t eat up your precious processor CPU and RAM.
Steps to install Gnome Flashback in Ubuntu 14.04
a) Open Terminal.
b) Run this command.
sudo apt-get install gnome-session-flashback
Wait for the installation to finish.
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Update:
If you are getting error as “Unable to locate package gnome-session-flashback”, try any of the option below:
Option#1: Run below command before running apt-get install
sudo apt-get update
Option#2:
Try launching Ubuntu Software Center directly to install gnome-session-flashback by clicking below link:
apt://gnome-session-fallback
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c) Save your work and Log Off.
d) In the login screen click the small white Ubuntu logo. 
e) Click the option – Gnome Flashback (Metacity).
f) Give your password and login.
You should be back to the Gnome environment. You will notice the good performance improvements over Unity.
