App Nap feature was designed to cut CPU and other computer resources to power-hungry applications when those applications are not currently in use.
It’s one of several OS X features that has helped increase the battery life on Mac portables, but unfortunately, this feature isn’t without downsides. If you’re noticing that applications don’t complete their tasks in a reasonable amount of time, then it could be due to resources for those apps being cut when they are backgrounded or left unattended.
To disable App Nap for certain app follow these steps:
Step 1. Go to Applications folder
Step 2. Locate application and press Command + I or right click on application and chose Get Info from the contextual menu
Step 3. In the Get Info window, check the option to Prevent App Nap
App Nap will no longer affect this application but not all applications offer this option.
To disable App Nap system-wide in OS X Yosemite check this tutorial.