The server lags a lot and it "cant keep up" a lot of times, reaching times like: 
Can't keep up! Is the server overloaded? Running 48931ms or 978 ticks behind

The hosting has 12 cores at 3ghz, 16GB ram, 400MBPS internet, 300gb SSD.

The server load is never going up to 30%cpu or 40% ram

Even with 2 players on it lags a lot. The server cas the cavesandcliffspreview Datapack

attaching logs.