Realms mob spawning is broken.

My friend and I made a mob grinder on realms(big box in the sky, gravity powered). It made only 5 mobs about every 20 min, even after we lit up all the caves. We gave up and made a zombie spawner powered mob grinder, still not very effective. We eventually cheated and added more spawners to the grinder, to make it run at the same speed it did when I backed up the world and opened it in single player.