In the latest 1.4.2 update, there has been a bug from a long time ago. Mineshafts that generate  start going crazy with halways that overlap and create huge amount of lag on some devices (Like mine Samsung Galaxy J3 2016) upon generating the area. Sometimes if there's lava  close to the mineshaft creates a LOT of fire and spreads to a lot of blocks which again, creates lag. Lag Doesn't go away until you stop the fire or break some of the mineshaft down, or even some of the some of the spawners(despite the fact they generate quite common in the mineshafts and they generate in qute the numbers).