Yes, performance is a known issue with the game but we are currently starting a big push on it, and it will improve significantly before release.
The game would definitely have performed much better at Rezzed, primarily because the computers were all connected over LAN. In it's current state NS2 puts quite a heavy load on the servers, so your online experience will be dramatically effected by server performance, which will in turn lower your own client performance.
The majority of the performance hit is on the gameplay code side, not as much on the graphics side, and it is much easier for us to focus on optimization now, as we've just gotten to a feature complete stage of the game. Over the course of the development there is a lot of fluctuation, as new features are constantly being added that can have huge impacts on server or client load, or both, so it really required nailing down all that stuff in order to properly work on ways of making all that stuff faster.
We've also put a lot of work over the past few weeks/months creating a robust set of profiling tools, to help us clearly see exactly what areas of the game are causing the biggest issues so we can really target the most problematic areas. We also recently just brought on a new programmer whose sole focus is optimization.
It can be quite a shock to go from playing the game on a LAN setup at a show like Rezzed to then playing online, but we've tried to be up front with people playing the game at those shows, so they know that getting involved in the beta stage of NS2 will be a little rougher, but to know that the game will improve leaps and bounds over the next few months. We fully expect it to be running as well as it was for players at Rezzed by the time we hit release.
--Cory