So, I am running a server on a dedicated machine which currently host around 13 players and I get a lot of performance issue on the server even with sufficient resources (8GB RAM, 8 CPU cores..).

• Server resources are not overloaded.

• It is not a network issue on the server end.

• People are not creating mob farms or doing anything that could explain the lag except  generating chunks while moving.

• A server with less than 8 players is running fine.

Console is showing the server as overloaded: 

[17:59:52] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 16430ms or 328 ticks behind
[18:00:25] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 18243ms or 364 ticks behind
[18:00:57] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 16869ms or 337 ticks behind
[18:01:31] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 19508ms or 390 ticks behind