Using Activity Monitor to check CPU activity

When an app isn’t responding or working correctly, it might use more of the processor (CPU) than it should, even when the app doesn’t seem to be doing anything. As the CPU gets busier, it uses more energy, which reduces the length of time that your Mac can run on battery power. It also generates heat, which can cause the fans in your Mac to spin faster. 

I’ve used this from time to time to see if an app was dragging the system down.