If you often run processor-intensive apps, it's good to keep an eye on your Mac's CPU usage. Luckily, Apple makes it easy to get a quick glance at CPU Usage and CPU History by changing Activity Monitor's Dock icon. Here's how to set it up.
First, open "Activity Monitor." If you don't know how, Spotlight makes it easy. Click the small "magnifying glass" icon in your menu bar (or press Command+Space). When the "Spotlight Search" bar appears, type "activity monitor" and hit "Return."
When Activity Monitor opens, you can close the main window with the red "X" button because you won't be needing it. (If you'd like to get it back later, select Window > Activity Monitor in the menu bar, or press Command+1.)
Next, right-click Activity Monitor's icon in the dock, and a menu will pop up. In the menu, select "Dock Icon," and you will see several options. For now, select "Show CPU Usage."
With "Show CPU Usage" turned on, Activity Monitor's dock icon will transform into a 10-segment gauge that lights up, depending on how much CPU activity is taking place. For example, if all 10 segments are lit, you're using 100% of your CPU capacity.
If you right-click Activity Monitor's dock icon again and select Dock Icon > Show CPU History instead, you will see a black rectangle that charts CPU usage over time. The display flows slowly from right to left, with intense periods of CPU usage adding more height to each column. The red squares represent CPU usage by system processes, and the green squares represent CPU usage by user processes.
If you'd like to change the Activity Monitor icon back to normal, right-click its "Dock" icon again and select Dock Icon > Show Application Icon. Alternatively, you can also configure Activity Monitor's Dock icon from the menu bar in the "View" menu. Select the "Dock Icon" option, and you will see a menu similar to the one we used earlier.
When you have the Dock icon configured how you like, just leave Activity Monitor running in the background as you perform other tasks. The CPU Usage or CPU History display in the dock will keep updating over time, and you can leave the readout going indefinitely. Activity Monitor must remain running for it to work, however; as soon as you close the app, its Dock icon will revert back to normal. Just like Cinderella's pumpkin!