I've noticed a lot of lag in both single- and multiplayer last few snapshots.
Slow mobs, sun/moon twitching, the basic lag you would expect on a overloaded server.
After some debugging I found this warning:
[Server thread/WARN] Something's taking too long! 'root.root.levels.TestWorld.tick.mobSpawner' took aprox ... ms.

So I figured to disable MobSpawning and it fixed the lag.

I couldn't find this bug posted anywhere, my apologies if I overlooked it.