Since Minecraft PE 0.12.1 beta build 7,mining speed messeds up...
For example, i can break the stone instantly with an NORMAL(Without any enchantments) IRON PICKAXE[I think now he breaks the stone in 0.25 seconds,like a golden pickaxe,which is more faster than an iron pickaxe]
Also other pickaxes are more faster,and i think this also happens with axes and shovels.Also my friend and others noticed this issue on different android phones...