is pc hosted on a server and are you playing over the internet or simply in lan?
if you are in lan then this is strange..
u must check a few things for me.. since i think that the server pc is okay try playing in that map with another pc (not the one u said you are using).. and see if u get all that lag.. i think it can be something caused by the number of monsters the pc must show on the desktop and if the pc you play on is without the correct gpu drivers it could lag with a lot of images to display together.. but i'm not sure..
to see whats the problem u must try.. i think server pc is okay anyway because it will start lagging if the cpu hit the max usage.. or if another periferic had some problems.. like for example the hardisk or the network card..