Sorry, I can't select the MCPE project - this is for the currently shipping MCPE as of early December 2014. I used to dig the bottom block of sand in a column of sand blocks, and it was very fast, the sand column would just collapse very quickly, i'd get a block of sand with every hit of my shovel, and I could collapse the column of sand blocks quite quickly.

Now it is very slow to redraw - takes a lot longer to break down a column of sand (by digging the bottom block repeatedly) than it used to.

If this was a gameplay mechanics change to slow down how long it took to digi sand, then ok, but if its a side effect of the shader tech, then it is a bug.