Is a single computer Application behaving erratically or slowly?

Quit the Application (quick and easy way is Command-Q) and re-open it. If it won’t quit, then you can bring up the “Force Quit” dialogue by pressing the Command-Option-Escape keys. Also, make sure the Application is updated…most Applications have a “Check for updates” option under one of the menus (in Microsoft Office Applications, it’s the Help menu for example). Some Applications are updated via the Mac App Store, under the “Updates” tab.