New in OS X 10.9 Mavericks and 10.10 is App Nap, a power-saving technology that looks at the applications that are currently being used and slows down the apps that it decides aren't being used.
Users can turn off App Nap in the application's Get Info pane (highlight the icon for the application and select Get Info from the File menu; right-click on the icon and select Get Info from the contextual menu; or select the icon and press the Command+i key combo). The Get Info pane sports a new checkbox in the General section named Prevent App Nap. Check the box and the selected app will run at full power all the time.
For Brief Client Users:
CLOSE BRIEF CLIENT, THEN MAKE THE FOLLOWING SETTING CHANGES:
Prevent App Nap for the Brief Client Application (Found in the Applications folder):