Server keeps lagging behind and I don't know why

I've added 8gb of RAM and my internet connection is fine. I don't see any way to give minecraft more CPU cores either. I still get the message

Can't keep up! Is the server overloaded? Running 2029ms or 40 ticks behind

Can't keep up! Is the server overloaded? Running 2029ms or 40 ticks behind

Can't keep up! Is the server overloaded? Running 2029ms or 40 ticks behind

Can't keep up! Is the server overloaded? Running 2029ms or 40 ticks behind

but I don't know why. I am running about 100 mods at the moment, I can provide a list if necessary.