Your Mac includes a basic diagnostic tool you can run yourself. To do that, you'll need to get familiar with Activity Monitor. Press the power button to turn on your Mac, and then immediately press and hold D on the keyboard. If your Mac’s Central Processing Unit (CPU) is overwhelmed by an app, everything on your system may slow down. Before switching up your favorite apps, you'll need to figure out which ones are slowing down your Mac. When you see the screen that asks you to select a language, release the D key. Follow these steps to do so: Shut down your Mac. Select a language, and then wait for the diagnostic tool to run.