OK, I'm going to give a definitive lowdown on what can cause any type of lag.
1) Older versions are buggy, and even if it doesn't lag, you should get the newest version (which is 2.0.4.) You can also get an updater program from srb2.org
2) Location has a lot to do with lag. It takes a lot more time for a chunk of Internet signal to go halfway across the globe then to the other side of your state.
3) Sometimes it's the server, sometimes it's the client. If pausing and using the listserv command several times doesn't help, typing in the ping command can sometimes help you see if a client is lagging.
4) Rather rarely, it's not lag at all, but consistency failures. If you have blamecfail set to on, and the console is getting spammed with messages, then that's what's happening. If it doesn't go away, temporarily turn off consistency protection and let the game kick the offender.
5) At times, your upload speed is just not enough to take the joiners...