Possible reasons would be
The server could be running out of ram space.
If you live too far away from the server, you likely wont have as good connection to those closer to it for example me my max is usually 4 bars.
Lots of really old useless plugins lying around. I dont have the complete lost of plugins but some which are not on use could possibly be taken away for the mean time like /comapny while its not on use atm, idrk if its being developed rn but its just an example. If some old lying around plugin could possibly be causing it that might be it.
Possibly limit the version of minecraft you can play in for example limit it to 1.12-1.12.2, if you allow too much i think it may cause some lag due to different versions being used.
Or the most common problem, too much entities, this includes Armor Stands, Animals etc... and yes those really stupid parrots and other animals just sitting in warp wild may cause part of it. And most especially slimefun can cause lag as it uses a lot of entities. That or the servers ram cant handle that much entities.
The servers tps should also be running somewher roughly around 20 ticks per second and ofc this occurs when players or animals move or even mobs. And some stuff most commonly know to cause lag, redstone. As well as crops perhaps.