[frollogaston]

I forget what heap setting I used, maybe it was 2G, but the old 2010 Mac mini I had as a server would lag if just one player was exploring land quickly (maybe by boat). Was online from 1.5 beta to 1.9 release, no more than 8 players usually.

[strogonoff]

I would say that without either setting your render distance to arm’s length, figuratively speaking, or allowing movement glitches and holes where terrain does not appear in time, “moving quickly while exploring” has pretty much not been a use case supported by the base game for a long time.

[frollogaston]

Right, or having some kind of entity-heavy autofarm, especially with version-specific bugs involved. Both things that in a moderately active server, someone will trigger.