According to Apple, its App Nap advanced technology feature in Mavericks helps you save power when you’re working with more than one app at a time. The system knows when a certain app is in the background, completely hidden by other apps’ windows. When that app isn’t doing anything, then, OS X will slow the app down, keeping it from using up CPU cycles, and thus battery power.
It’s a great feature, and one of the reasons why your Macbook may seem to have more battery life than it used to, thanks to Mavericks.
When switching to a napping app, it can seem to take a couple of seconds for OS X to get that app back up to speed. If you want to reduce the start up time of a napped app, you can disable App Nap for it.