You may need to activate a different video Driver. Ubuntu comes with a fallback video driver that does run the desktop slowly. The fallback is designed for older machines that do not have video cards that can run accelerated graphics in the video card. Sometimes the fallback video mode will run when we do not need it. For example, it will run if we boot into Recovery Mode>Resume.

Go to System Settings>Software and Updates>Additional Drivers tab. Allow Ubuntu time to find some drivers for you and then experiment.

Regards.