I installed Xubuntu desktop on top of Ubuntu 12.04. The usage of RAM and CPU is in general lower than in Ubuntu. But when it comes to some serious computations, I did not see much of a difference between the two.
For example, I ran a MATLAB program that took 99 seconds to run on Ubuntu and 98 seconds on Xubuntu. Then I ran Geekbench benchmarking and the overall scores Ubuntu and Xubuntu are 2996 and 2997, respectively.
My laptop is a 4 year old Dell 1535. Would things run faster on a clean install of Xubuntu?