I'm having similar issues with a world suddenly becoming unplayably slow. I'm using 0.13.1 on Android tablet. Others are also having this issue as seen here: http://www.minecraftforum.net/forums/support/minecraft-pocket-edition-support/2565715-my-world-suddenly-unplayable-extremely-slow
Is there a way to compress/rebuild/reindex the worlds to get the game play speed back to normal? Would it help if I could attach the world here?
Seeing issue on 4.4.2 and 4.4.4 version of android tablet/phones.
This was happening for me on MCPE 1.8.0. I closed the app and cleared the cache within Android settings and I was able to restart the download.