Minecraft PE is Lagging

As said in the title, The game lags when I play.

 

Steps to reproduce:

1. Open Minecraft
2. Select a world
3. Play normally
4. The game lags how much ever depending on the world I was playing

Observed results:

If it's a super flat world, it lags a little. If it's not a super flat world, it lags a lot

Expected results:

It shouldn't lag at all no matter what world I am in