Mobs now stack, it means instead of having 100 zombies walking around in your dark room, you have only 1, but with a X100 on top of it, it means you can kill it 100 times. This will prevent lag on some island when their owners just go AFK all night. Also, if you are too far or too close to your dark room, mobs wont spawn.
This plugin might slow down some AFK farms, Solution: you just need to boost your creativity and redesign it, there are a lot of ways to make mobs go wherever you want (And I'm not talking just water)