App Nap

App Nap helps you save power when you’re working with multiple apps at the same time. macOS can tell when an app is completely hidden behind other windows, and if that app isn’t currently doing something for you, such as playing music, downloading a file, or checking email. If the app is hidden or not doing something, macOS slows the app down. As soon as you start using the app again, it instantly shifts back to full speed.