ggogle for the listenserver, and uhm, well.
You don't get lag because you're essentially on a LAN, everyone else however.. ='/
Try limiting teh player numbers, 8 or even 6.
Don't put on large maps, and try not to have any crazy mods running.
Also make sure that ALL you are runnign is the game server. no other programs, turn off your firewall (windows one too) and double check you're not runnign anything else.
That's teh only way to reduce lag, apart from getting a decent server and paying for it.
I intend to get a seperate tower for use as a server verysoon.
So I could help you out there.