I have a world with a lot of multiplayer player data (eg the data that saves a player's inventory, last coordinates, name, etc). All the data is slowing down the game only in this world and makes it freeze when players join and drop to around 5-10 fps when a player is on my world. I've had maybe ~300-400 people join this world and the data is slowing it down somehow. Even opening the world can take ~30 seconds or even just crash the game. This has been an issue since 0.11 and still is an issue in 0.15.2. The world is 33.3MB superflat. I'm not sure why this happens, it's like mcpe is reading player data when it doesn't need to do it...