I wouldn't say that UT2K3 is fast because it wanted to serve as a reminder of the good old days (We don't lie when we say that you could run 90 MPH in the original DOOM.)Rather, I think that in Epic's mind high speed is simply more appropriate in a multiplayer game than a single-player one.
In the latter, the player needs to be immersed in the gameworld. Soaking up the richness of the story, enjoying the sweet eye candy, and realizing the various tactical opportunities presented are all more easily done by slowing the player down. Some single-player titles also use slow speed to pace the game much like in movies, adding a dramatic effect.
In the former, the competitive nature of the game tends to focus all of the player's attention on the gameplay alone. Stopping to enjoy the view will get you shot or left behind for example. The same goes for a player taking his time to work through a strategy or the story/theme of the game. Time in a multiplayer title is in a sense no longer your sole property but shared among others.
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