Significant block lag on worlds created in these snapshots. Block lag seems to be equivalent to the amount of time it takes to break the block - seems like Minecraft does not register that I am "punching" the block until the block breaking animation has completed on my display. Oddly enough, no block lag when playing worlds created in older versions with a client that is updated to the current snapshot.

My procedure was as follows:

Download MineCraft
Update client to latest snapshot
Create new world
Break any block