I am not sure if all these factors are actually relevant, but here we go:
- /gamerule mobGriefing false
- Hard difficulty
- Villagers inside a house
- Zombies trying to break in (difficulty is set to hard, so they would try to break the door, but /gamerule doesn't let them)
These things MIGHT be related to the issue. I get really bad delay on picking items, killing mobs and even brewing.
NOTE: this is on SINGLE PLAYER, clearly an internal server problem.
NOTE2: when I kill all the zombies, the delay is gone. There is probably something wrong with the new zombie system as well.