If you are still running iOS9 we would suggest you to upgrade to the current iOS version and avoid potential problems.
iOS9 has some memory problems that we can not go around and fix.
As a temporary fix if you are experiencing crashing apps and slowness of the device, you can do a hard restart of your device by following the procedure :
- Just press and hold both the Sleep/Wake and Home buttons simultaneously for at least 10 seconds, until you see the Apple logo. You can let go of both buttons after the Apple logo appears.
- Your phone will go through the usual process of starting up.
- You'll be back in your home screen.