OS X El Capitan: App Nap
App Nap helps you save power when you’re working with multiple apps at the same time. OS X 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, OS X slows the app down. As soon as you start using the app again, it instantly shifts back to full speed.
Last Modified: Apr 13, 2016