ake sure you have UT 2004 configured for a broadband connection for cable or DSL.
It's normal to lag for a minute when a map first starts. During that time, a lot of data is being shoved through your internet connection. So after about a minute, the lag should go away.
Computer lag is when your computer can't display the graphics on your monitor fast enough to play smoothly. This is called your framerate (FPS - Frames Per Second). A framerate below 30 FPS is considered "laggy". You can see your framerate by pressing the Tab key and typing stat fps. To disable it, just type stat fps again.
To fix computer lag, you can do a few things. The quickest and easiest way is to turn down your graphics details. Do this by pressing Esc key, click Settings button and uncheck all the boxes and set all the options to Low or Lowest. The graphics will look worse than before, but at least you won't lag. As you play, you can gradually turn up the detail until you find a good balance between lag and graphical detail.
Another cause of computer lag are other programs running on your computer. Programs such as web browsers (Internet Explorer, Mozilla, Firefox, etc) can easily use more than 50% of your CPU even if you're not using them. So close your web browsers and any other programs that are running. This will free up more CPU to be used by UT 2004, which will fix the lag.
If you want the pretty graphics without lagging, you'll need to upgrade your computer. The best place to start is with your memory. People who play UT 2004 usually need at least a gigabyte. If you have less than a gig, you can expect long load times and computer lag. The next best upgrade after memory is your CPU. And after that, your graphics card. If you're willing to upgrade all three, you might as well buy a new computer to save yourself some money.
The other type of lag is network lag. This is harder to to fix. Network lag is usually caused by excessive use of your internet connection, such as running eMule or BitTorrent in the background. Network lag can also be caused by your ISP (Internet Service Provider), or if the network traffic from your computer to the server is being routed through slow or congested network equipment.
To fix network lag, close all internet-based programs, such as file-sharing programs and web browsers. If you have a broadband connection (cable, DSL) and/or a router, you can try power-cycling (unplug power cord, plug it back in after 1 minute) both the modem and router. If you know how, you could even try getting a new IP address, though that probably won't help. And lets not forget to reboot! After rebooting, don't open any other programs except UT 2004.