Hello, I have my small snapshot 19w05a server where we playing in only in 2 - 3 players at the same time. Nobody generates new chunks, and server sometimes has big lag spikes and rollbacks.  Console constantly saying "Can't keep up! Is the server overloaded? Running 3052ms or 61 ticks behind" But CPU of server and Total ram doesn't go over 50% with all three players.

startup .bat file:

@echo off
java -Xms2G -Xmx4G -jar "server.jar"
pause

Sometimes the server puts players back, and say in console "xxxxx Moved too quickly" but the player is only on the boat.

MC server has 4GB of allocated memory.
Physical server specs:
CPU is 4c i5-3470 3,20GHz / boost 3,60ghz
RAM 6GB
64 bit OS Win server 2008 R2
NET: 100Mbit up / 100Mbit down (real +-10MB/s up-down)