I have been experiencing lag problems in a small network. It took some time to work out what seems to be the problem. If someone is running a server (non-dedicated) with bots and has the graphics set to high, the LAN game is very choppy. The problem is he doesn't actually notice this because it's fine on the server. If he reduces the effects he doesn't get any noticable performance increase but the network does. It would help if the server user was at least informed if his server is running to slow or the graphics-priority was reduced IMHO.We had the problem on fairly low spec machines 1.8 GHz or so. Works fine though with effects turned low.
I think I have experienced it online as well though, as my machine at home (2.6GHz Celeron 1GB RAM & 256MB AGP Graphics card) also has choppy graphics with some servers. (okay, Switzerland often has large pings, but I don't think that's the problem)
(Hope this is more helpful than obvious)
Moermel