In the upcoming Ubuntu 12.10 - Unity 2D will be removed. Thanks to LLVMpipe, a computer that isn't capable of running Unity 3D is getting the same experience without the Compiz effects.
But what if I don't want Unity 3D at all? I have a Dell Inspiron 1525 that's capable to run Unity 3D, yet it's slow. Very slow. Unity 2D, however, is faster. How do I enjoy the snappiness of 2D from 12.10?