Many common software and hardware issues with the iPhone such as camera malfunction, slow scrolling, app crashes, and more can be resolved by following the steps below:
1. Make sure your phone is updated to the latest iOS software. While connected to a WiFi network go to Settings/General/Software Update to make sure you are using the latest software.
2. Attempt a “soft reset”. Hold the home and power button of the phone for approximately 15 seconds until the phone restarts and you see the white Apple on the screen.
3. It's also possible that a faulty app is causing this problem. If you've noticed that this has started happening after downloading a specific app then you should try deleting that app and see if the the problem continues after deleting that app.
4. Attempt a “hard reset”. Be advised that this reset will delete all of the data on your phone so before resetting back up your phone using iTunes or iCloud if you want to keep your data. Go to Settings/General/Reset/Erase all Content and Settings.