Mob spawn rate too high

I run a snapshot server, so roughly with around 6-7 people on, the servers TPS goes to 10-11 when the servers difficulty is easy. When it's put to peaceful it's at 20. 

While normally I would call this server lag- In this case I think it's the over amount of mobs spawning, in a span of 5 minutes of mobs being turned on, one of my players were surrounded by 12 skeletons, 3 creepers, 2 endermen and several zombies. This then ended up crashing our server because so many mobs just continued spawning. 

Edit: I've done further testing. http://prntscr.com/npjmx9 

This is with only one person on, I added a tag to all mobs that spawn and then turned off mob spawning and this is what I've got, I've done this a few times- It goes anywhere around 1250 to 1950, to sometimes somewhere in the 3000, This is not okay.