Mob spawn rate too high

The normal rate is  for hostile mobs is

70 * [number of distinct chunks loaded by any player with a max of 289] / 289

Which means the more players are scattered around the world, the more mobs could spawn. This does not depends on the difficulty level. Actually the spawning rate was underrated in previous versions due to several issues, and the last one was MC-150218. It works now as we would expect. When in multiplayer, this total cap is of course distributed between players, but you could end up with one player having the "mischance" to receive a larger part of the cap than the others.

Your issue is not really about mobs over-spawning, but more about performance with mobs in general.

12 skeletons, 3 creepers, 2 endermen and several zombies

That's not really an unusual amount of mobs. I'd rather say, that's quite normal.

Reopened, multiple other people have reported exceptionally high spawn rates as well.

However, I believe that the spawn rate just appears to have increased that much because MC-147890 has been fixed.

I set my server difficulty to easy to try to cut down mob spawning and increase performance. There are definitely a lot more mobs in PR2.

Like I said, the difficulty level does not affect the number of mobs that are spawning unless you are on peaceful. Only the raid, the damage amount the mobs are dealing and their gear will differ. Mobs count could have been perceived higher before they fixed issues related to mobs failing to spawn on corners of chunks MC-150218 and also not despawning beyond 128 blocks. MC-149420 

A proper mob count has to be done and to compare against expected values to check if this issue is valid.

I have this issue in 1.14.1 pre-release 2

still happening on 1.14.2 pre 3 and 4

So it's fixed in 1.14.3 pre release 1. but the spawn rate is too small for me in my own world

This issue is definitely still happening. First noticed it while I was holed up in a Shipwreck, and heard a literally non-stop barrage of zombie groans and drowned gurgles around me. I went into spectator mode because I was confused as to why I could barely hear the other members of my realm over the noise, to find literally hundreds of mobs had spawned in the cave system below me. The rest of my members had also been struggling to play for the entire day (we have about five people online at any given time, at about this point, we only had three) due to lag and rubberbanding.

Later, when reaching shore again, I found another huge batch of monsters just standing around on the surface waiting for me and quickly switched into spectator mode again. Even later, when I found a village and night fell, I witnessed an equal or greater amount of mobs spawning in the village, so much so that the iron golems were easily overtaken and groups of dozens of zombies were following around the poor defenseless villagers.

The lag was so bad that the mobs all take maybe one or two steps before freezing in place, every single time they move. Players walk for maybe a chunk before being pulled back several blocks. Flying with an Elytra is all but impossible!

My realm has been updated for 1.14.3 just within the past couple days, and we didn't seem to have this issue as badly before, though my boyfriend has complained of being occasionally ambushed by large groups of mobs out of nowhere over the past several days. I understand the need to increase mob rates for those who have been having issues with getting mobs to spawn, but I think this has gone entirely too much in the other direction. I hope it gets fixed soon, my friends and I would like to play without constant rubberbanding!

This might just be mobs that spawned in 1.14.2 and haven't despawned yet. Please change your difficulty to peaceful and back again, and check if the issue persists.