On Ubuntu 12.04 LTS, I installed VirtualBox (last version) where I am running Ubuntu 12.04 LTS and Ubuntu 14.04 LTS.
I noticed a significant difference in terms of performance: mainly, the speed. All the tasks I run on Ubuntu 12.04 are faster. I want to do most of my daily work on Ubuntu 14.04 but every task seem slower. I tried them on Vmware Player but the difference is still the same.