App Nap helps you save power and battery life when you’re working with multiple apps at the same time. You can disable this feature for each application by using the info window in the Finder (select the app in Finder and activate this window with cmd⌘ + i). With the following command you can disable App Nap system wide.
defaults write NSGlobalDomain NSAppSleepDisabled -bool YES
Nap App is immediately turned off, to revert to the original settings use the following command:
defaults delete NSGlobalDomain NSAppSleepDisabled