Other Players lag on my self-hosted server.

When I have my server up on my 2nd computer, all players who join lag if *not on a superflat world. I only have the server itself active, and I have 5 - 6 GB dedicated. Furthermore, I do not lag, and the server is not becoming over loaded. I am aware that I am not lagging due to a local area network, and the lag may be caused by my internet provider, but just incase it is a bug, I made this issue. Thanks.