Strange Lag Issue..

[Dylxn97]

Hi guys! Bit of a strange query here that I'm hoping one of you wizards can help me out with:P
So I've made a server, my server runs fine as tested by me, and a friend in a different part of the country, happy days right? No:(

So my server is fine until I join, then everyone gets constant connection interrupted lag and the server basically becomes unplayable, but if I'm not there I can have 18 players connected at less than 100 ping.
Not sure what the issue is here, anyone have any ideas? Thanks in advance!

[Xerxes]

Are you hosting an playing from the same machine?

[Dylxn97]

Xerxes I'm playing and hosting on 2 different pcs, connected to the same network