Read this how to disable App Nap:
http://www.tips-and-tricks-in-mavericks.com/disable-app-nap-on-a-per-app-basis-in-os-x-mavericks/
or
Originally Posted by Apple
Note:*If you experience problems with App Nap, you can temporarily disable it for a particular process by typing:
defaults write <app domain name> NSAppSleepDisabled -bool YES
