I am currently running a plain Vanilla Survival server for myself and some friends.
Late in the evening it is usually just me online and the server runs with minor hitches,
but as soon as multiple people are on the server just can't keep up and says so in the command line excessively.
Now I can't imagine us putting so much stress on the server as I know there are
no intricate redstone contraptions running unsupervised and there is no excessive buildup of mobs anywhere around spawn.
Basically even just building on the same site with two people has the server in its knees with the 3gb RAM almost topping out.
There are only two data packs installed: Vanilla tweaks No Enderman Grief and Vanilla tweaks One Player Sleep.
The version is 1.16.4 btw.
I'll gladly provide more info if needed. currently at a loss on what causes this or how to rectify it.