Massive Lag with water(/lava?) flow

I use flushing mob farm. But once i activate them they start making massive lag since 15w49. FPS doesn't fall while lagging but mobs and actions start lagging like less than 1fps.

I'm thinking water flow (or new pathfinding) causes lags but not sure.
At least there are lots of water flows when they start lagging.

This never happens on 15w47 or earlier.