Hello everyone !
First i am playing on a Galaxy Note3 (6.0.1 custom rom) And Nvidia Shield K1 tablet (6.0.1 vanilla). I have same problem on those devices with minecraft vanilla (no mods, no hacks, no root) on the very last version available 0.15.1 (i do updates).

I have a big problem with my save. Actually i have found two close mob generator and created a double experience farm structure like we can find here and there on youtube. The problem is that i afked the game near those spawners and mobs has been stacked for some minutes. When i came back to the game i couldn't attack anymore because of lag.
I tried disabling all fancy options to reduce the lag and even managed to remove an enchant table (with booshelf arround) and a nether portal that were nearby to minimize visual effects. At first i was playing on my phone but i copied my save game to my more powerful tablet but it does not change anything.

When the game is lagging it only recognize long press on screen (i can only destroy blocks). But if i tap to the screen to attack it recognize it as small movements arround. It does not attack. I even tried to pair my Shield Controller to use the gamepad button to attack but still no effect at this state of the game. So i couldn't reduce the number of mobs by my own.

Finaly i tried to move very far to change chunk so maybe all those mobs would be "deleted" after few time but it did not change anything (or i did it wrong?). So i came back to the place and here is the save game :
https://dl.dropboxusercontent.com/u/15055068/9RcBAF7rAQA%3D.7z

Please tell me if you need more detail.