Is a single iPhone/iPad/iPod/AppleTV App behaving erratically or slowly?

Quit the App by double pressing the Home button and then sliding the App upwards (on iPhones without a home button, swipe up and to the right from the bottom of the screen, then slide upwards on the App). Also, make sure the App is updated by visiting the App Store and the Updates section (pull down to refresh). Newer versions of the App Store require you to tap your avatar in the top right and then pull down to refresh updates.