I forgot to mention that the devs are working on adding an option to 'scale up/down' the number of hostile mobs allowed (possibly a scale to mob density). They, however, mentioned this quite a while ago, and it has yet to be implemented. It, in theory, would greatly help, but even with a higher mob density will overtime surcome to the same issues that we currently are experiencing. Mobs just don't despawn due to the stupid rule that they can't despawn if they are in a dark location.
The main reason for the low density right now is due to some devices not able to handle the additional load. This is and always will be an issue when using apps that are designed to run on many different types of platforms, in essence, those of us with more powerful platforms suffer to the whims of those using weaker platforms.
Anyhow said my peace, best of luck.