This has been an ongoing issue with X11 compositers like marco, unity and cinnamon etc... Ifind especially with an Nvidia GPU that as the system runs longer after a number of weeks or a number of months the GUI just gets slower and slower. It's not something you'll instantly notice but just a hint of lag here or there that gets worse.
One hint you'll find is by watching nvidia-smi and checking the utilization you will start seeing random spikes of 80-100% even without maxi........
This was done on a 2900 but applies to all the switches of the same era.
Step 1 - Power Cycle and enter recovery mode
If you have physical access you can power cycle and hold the mode button down for 15 seconds. After that the SYS light will flash on the switch and you will see the following screenshot.
If you don't have physical access (eg. it is a datacenter swich over console only) then power cycle and hit "Ctrl+Pause/B........