18w47a is extremely laggy
 
I'm having a LOT of issues with lag (single player) on this snapshot. Loading my world from the previous snapshot in this one for the first time took nearly a minute, during which my character was standing on the edge of a cutoff where nothing would load in, and moving made me fall through the world repeatedly before it finished loading.
So far I've had lag issues with breaking and placing blocks, using a hoe on dirt, and hitting mobs, as well as mob movement, leaves not despawning/dropping items despite nearby logs having been removed, farmland not turning dark/saturating from water, and saplings and planted food not growing. The mob lag is bad enough that fighting them is almost certain death (on normal).
Most of the non-mob lag is much less disruptive, except that the lag in breaking blocks can sometimes cause blocks to reappear and then stay as if they'd never been broken. 

ETA: Noticing that the sky also lags and reverses a small amount every few seconds, and at least once now I have completely rubberbanded on a boat, shooting back at least 10 blocks or so. The water-droplet trails when a fish is approaching your cast fishing line, will stop/disappear for a moment before resuming their movement. Rain also makes all lag worse, as could be expected.