Minecraft 15w49b, Minecraft 15w50a, Minecraft 15w51b, Minecraft 16w02a, Minecraft 16w03a, Minecraft 16w04a, Minecraft 16w05b
see crash Report and Screenshots
Tested in new world and old world.
Problem happens earlier in old worlds.
I tested it with this Command:
Commandblock is in repeat-mode (activated via Redstoneblock).
In old worlds you need only 5 Commandblocks running this or any other Command like /testfor @e - in new worlds it needs up to 20 Commandblocks.
It needs vom 30s to 5 min to have those lags.
When the lags appear Mobs seem to be in slowmo.
commandBlockOutput is false