You know when have all those spawners and all this mobs just creating lag? Here is a fix.
When more than one mob spawn they will stack, so if two blazes spawn, instead of two entities, it will be one entity with a name tag that says "X2". When you kill this mon, it won't disappear, it will just turn into one blaze. Loot will still drop, and lag will be gone.
Someone will eventually mention it, villager converting. Well, what happens when a zombie villager spawns? Well, in order to stop massive villager converting, and glitches, zombie villagers will not stack.
Also, I do know this is possoble, as I have seen it on a different server.