Server side lag: how to improve

Sorry if there is already a thread about this elsewhere on the forums; I guess I haven't looked hard enough. My server uses forge 1.12.2 with only one mod: The Twilight Forest. While this is a pretty big mod, it shouldn't slow down the server too much; not to the levels that I experience. We only have 5 players on at a time, but there are big slowdowns, with frequent rubber-banding and glitchy block breaking. One time a player turned invisible; no one could see him. I believe this is server side, but let me know if you think I am wrong. I don't think I could get any plugins with forge, so I don't know what to do.