I marked this as a bug, even though i think its an intended mechanic within spigot, and it baffles me why it is so. 

Right now, it seems you have a total mobcap, world wide (world, nether, end etc is separated, indeed this is a good thing)

but, when it comes to the mechanic itself, its kind of broken. we are a small server, we tend to be max 3-5 players on, many afk over night etc. 

So the way mobs spawn now, as i said, a total mobcap, you set your cap, thats it. at least this is how i see it, ive done no counting of enteties, but i can see it with my bare eyes what the problem is.
its not just affected by people standing afk, it could be a person mining, hugging all the mobcap, this hurts people with mob farms greatly. we, who have been running around torching everything, to optimize our spawn rates, gets hurt by this.

as an example, i am in a mushroom biome, a large island where no mobs can spawn. in that island is a squared spot, this spot is an ocean biome. clearly i made this my darkroom mobfarm, because i would not need to torch anything, everything where optimized from the start. if im online alone, or with others with optimized farms, this works great, but as soon as someone logs on, that is not killing off mobs, they start to hug the whole mobcap, to the point where there are NO mobs spawning in our mobfarms.

as i said, im not that technical, but a solution would be to at least let people set a player pr player mobcap, and a separate total mobcap for the whole world.

lets say, i set 500 as the total mobcap in the settings. then 50 mobs max pr player. there would be max 50 mobs around me at any given time, even though the mobcap is set to 500.
player 2 comes on, he spawns 50 mobs, now there will be 100 active mobs at any given time. if lets say we then exceed 11 players, now we have reached the total mobcap, and now only 45 mobs can spawn pr player, and already spawned mobs that are over the now new mob cap de spawns to count for this. 

Totaly ok to have your defaults as is, but let me be able to change player pr player mobcap, or at least let me set it to vanilla spawning mechanics. 

increasing the total mobcap does not help, it reatches total mayhem, and still one player can hug the whole mobcap.

we now have a scheduled task killing off common mobs every 10 minutes, i don't want to do this, but this is the only way we can have some mobs spawn in our farms.

this breaks the game and the SMP feel we all love so much