We are seeing game freeze ups every hour or so.. The game looks like its working, but on freeze we've seen various game crash indicators like:
containers cannot be opened
mobs are frozen
game sounds not playing (breaking blocks, or walking sounds, or mob sounds
crafting does not work
minecarts stop travelling
and mixes of all of these.

We also have not seen illager patrols in the last two betas, even starting new worlds.

In the nether, we tried building a wither skeleton farm in the middle of a nether fortress, and even though we are seeing natural spawns in the nether, we saw almost no spawns at all in 31x200 of nether brick.

We created a few wither skeletons spawners to make up for that after a few hours of testing, and the wither skeletons could not see over stairs or even dark oak wood trapdoors.

In addition, villagers seem to get stuck on top of chests, and cannot get off to path to their crafting stations. (Pushing them off, they get the green aware sparkles, and then path back to their crafting stations.

Performance wise, the game does feel faster and smoother.