Spigot activates or deactivates entities as they come far away. This greatly saves on lag. However, it does not have an API for plugin developers to do the same!

This could save a lot of lag for developers - For example, if a player wanted a villager NPC who traded with people, he could de-activate the NPC, reducing any lag created by it.

It's a thing I personally really need with Spigot, and I think it would be useful for other people as well.