Game became almost uncontrollable due to huge lag in particulal area

Hello,

My survival world suddenly became unplayable due to lag in certain area around my initial spawn point where all my stuff is constructed. It lags so much that most time I can't even turn my player or place any block - any touch is interpleted as long touch. But I can move using arrows and when I move far enough from the area (I think about 60-80 blocks) lag dissapears and all works ok, just I return - again lag.

It happened when I was trying to speed up my XP farm (using zombie spawner underground at about level 10) and killed a big slime (there is also slime chunk there). I thought it's because too much zombies spawned, but after hard tries I turned off the spawner and killed all zombies, but lag had not disappeared. Climbing up from the underground didn't help too.

I tried to load/unload the world and the app several times. Two my other creative worlds works fine.

Providing all my Minecraft PE files from iTunes so you can reproduce the bug.

https://drive.google.com/file/d/0B7AO0v4hK3VgbU5zNGpkS0NpZmM/view?usp=sharing

Please help! Don't want to loose a lot of work

Thanks in advance.