Un-playable lag on a server with 4 GB of ram!

When players log on to the server, they get massive internet lag and then get disconnected. But only client side! They are still on the server for me and the console doesn't say that they have disconnected, so then they log back in and see themselves! The error that they get is
Exception: io.netty.handler.timeout.ReadTimeoutException

This is making it impossible to play on my server.
I have tried re-starting my router, resetting the server files and they have tried resetting .minecraft