I would definetly check and see if you need to install any proprietary driver, which is not Open-Source. If the driver isn't installed, there is no way for the OS to communicate with the GPU. But Since the CPU is, the systems utilizies the CPU to calculate real-time image rendering (What you actually see on the screen). This causes the CPU to be busy all the time, while still being in the need to calculate different algorithms.
To check whether the GPU (Graphics Card) Driver is installed, the easiest way is (if you are using Ubuntu with Gnome, not Unity) to check your bar (where notifications and the menu, window manager,etc. is located), if it indicates a light shadow beneath it. This means, that Compiz is activated, producing a nicer appearance of mostly everything. Since this means higher Processing Consumption, the System needs a stand-alone Processing Unit, like a GPU (Which basically is a so called Co-Processor), to enable enough Processing Power.
If it doesn't happen to be, follow the very first step I mentioned.
Hopefully helped out