I have noticed over the last several versions that if I leave Illustrator running for more than a few days without quitting (with and without any files open), that my system gets less and less responsive, sometimes to the point where it seems completely locked up other than cursor movement. A quick mouse over the Illustrator Icon in the dock shows that the Application is Not Responding. I have also noticed the same thing happens in Photoshop. I'm not sure if there is a memory leak in these apps or in the Creative Cloud Core environment somewhere. I took a look in the Activity Monitor and it doesn't seem to provide me with enough data to make a determination. Quitting the apps will usually return the system back to a stable fully functional state.