In the new snapshot, attributes were added, I assume that this includes speed as well as attack and health. Zombie Pigmen were too fast before snapshot release(and were fixed), but Baby Villager Zombies weren't fixed. If this was in survival, it would be OP for the zombies and the player/villager would have (almost)no chance of living.
In the pictures below are images I managed to catch of the uber-fast baby villager zombies. You can guess how fast they are going by the width of distance between their legs. No speed potions or external editing was used to make the baby villager zombies faster.