The game becomes significantly laggy after temporarily locking the device, and the only way to recover from the state is to close and restart the app. This seems to affect both iOS and Android devices.

Steps to reproduce:

1. Open a world.
2. Lock your device by pressing the hardware lock button.
3. Unlock it.
4. Now MCPE is in the paused state. Resume the game play.
5. Observe that it's laggy now.

The problem can be worked around by closing the world before locking the device. Merely pausing the game does not do the trick.

See also: iOS Stutter : MCPE