Zombies cause lag. The lag does not affect the FPS, however it effects the game ticks because it causes the player to not cause damage to the zombie. I'm not sure if this is because they are customly spawnned zombies. The lag goes away when I change to Creative, meaning it has something to do with the Zombie's AI. This is the command I used to spawn these zombies in.(MultipleZomies are spawnned: 5-15)

/summon Zombie -1039 104 1871 {CanPickUpLoot:0,PersistenceRequired:1,CanBreakDoors:1,Attributes:[

]}