How to fix Server Lag

There are multiple ways. Do all of them for the best riddance of lag!

Uninstall all useless or not used plugins and skripts that might be taking up memory on your server.
Install Paper, it helps with managing lag
Install the clearlag plugin to clear dropped items every 30 seconds/1 minute
Set a limit on how many players are able to be on the server at one time. A lot of players can overload a server!
Try to reduce the amount of packets being sent to the server by reducing the amount of boats!
Make sure to restart your server at least every 24 hours