I did a bit of testing in the snapshots and on average 59 Endermen would spawn in a second after doing /kill @e[type=enderman]. I had about 14 chunks of warped forest around me and I was on EASY difficulty. Then I tested the same thing on hard and the average was 784 Endermen. Which was crazy. I hope this gets fixed because that's very hard: It's on-average more than on the end mainland in all difficulties.