The server was facing some bad lag today while there was only about 7 people on. I took a bunch of screenshots of me using /lag and will post the results here in an attempt to help in finding the source. pay attention to time stamps, RAM, and entities in the 1.13 world.
as you can see the server very sporadically spawns 2000-3000 entities and maxes out ram and then abruptly they all go away. in the last image you can see that RAM usage goes from maxed out to fine in a matter of seconds.
My uneducated guess as to why this happens is it has something to do with elite mobs. when they spawn they have tons of particle effects all over them and they often spawn with a large number of minions aswell.
hope this helps!