Is it possible to change Unity 3D to 2D and will I gain any performance boost after that?
Unity 2D is not available in Ubuntu 14.04, thus you can't switch. The packages for unity-2d in 14.04 are just transitional dummy packages. They don't contain any files but depend on Unity 3D.
Cannot promise performance boost as such, but see if this can be of any help (from https://sites.google.com/site/easylinuxtipsproject/speed ).
Disable the visual effects: switch to a 2D desktop environment
- By default, when your video card can handle it, the 3D visual effects are enabled. However, these may cause malfunctions or sluggish performance. You can disable them by switching to a 2D desktop environment.
There are several options for this.
My personal favourite is a switch to the Xubuntu desktop. But there's another option as well:
a. First, install gnome-session-fallback:
Click on the grey Ubuntu logo (Dash home). Query: terminal. Click on Terminal. Type (use copy/paste): sudo apt-get install gnome-session-fallback
Press Enter. When prompted, type your password. Your password will remain entirely invisible, not even dots will show, this is normal. Press Enter again.
b. Log out. In the login window, click on the Ubuntu logo next to your user name:
Then click on GNOME Flashback (Metacity). Note: don't select GNOME Flashback (Compiz), because that also has 3D effects, and you want to get rid of those.
c. Log in again.
Unity 2D is not available in Ubuntu 14.04.
However http://lubuntu.net/ is a lightweight flavor of Ubuntu, currently available at revision 14.04. It will run far faster than even Unity 2D.