Utilize Activity Monitor


Find out what's wrong with Activity Monitor

Activity Monitor application is not only a great tool to see what is going on your Operating System but also a useful tool to exit a troubling process (Force Quit application).

Start Spotlight by clicking the magnifier icon at the top right of the Desktop
Start type act and Activity Monitor will show up. Hit Enter to launch.

In the Activity Monitor, select "All Processes" from the View menu

View menu → Display Items

Check Real Memory is selected

CPU, Real Memory, each column header will let you sort (indicated by V as shown). Check what process is pumping up CPU and/or Memory