I have been running a small server for my friends and I for about a month without issues.  After updating from 1.13.1 to 18w43b certain rivers will spawn hundreds of mobs in close proximity.  This results in horrible performance, with the server using over 8GB of RAM for a single player in the affected area and the server falling behind as much as 250 ticks.  The picture provided is after killing approximately 100 of the spawned mobs.