C'est deux choses, des modifs dans le scheduler pour jouer dynamiquement sur la prioritée des taches:

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 — playing music, downloading a file, or checking email, for example — App Nap conserves valuable battery life by slowing the app down. But as soon as you start to use it again, the app instantly shifts back to full speed. It’s so seamless you'll think it had been running that way all along. App Nap works the same way with Safari tabs, so only the visible tab is running at full speed. With App Nap, CPU energy use can be reduced by up to 23 percent.1

Et compression de la RAM en cas de besoin d'une application:

Room to run.The more memory your Mac has at its disposal, the faster it works. But when you have multiple apps running, your Mac uses more memory. With OS X Mavericks, Compressed Memory allows your Mac to free up memory space when you need it most. As your Mac approaches maximum memory capacity, OS X automatically compresses data from inactive apps, making more memory available.

Extra fast.Once the memory is compressed, your Mac doesn’t have to waste time continually transferring data back and forth between memory and storage. So it’s able to get more done in less time. And since compressing and decompressing happens almost instantly, the only thing you’ll notice is how responsive everything feels. Especially when you’re doing everything at once.