Many of us have found that ustream lags when you are using the full ustream page and chat. However, we have had excellent results by not using the ustream page, and instead either popping out the screen separately or just using the Dashboard AND using IRC for the chat. I have an older Mac (2004 model) and I use Colloquy, an older IRC chat program. I connect to ustream chat room by typing in the name of the show. I get no lag on the chat nor on the video. It's really the whole ustream page that lags because there are ads on it and other flashy features that you don't need to see when watching the stream.
If you can't be bothered to set up another IRC program, you can also fix the lag temporarily by reloading the entire stream page every so often. It doesn't last for long, maybe 1/2 hour, before it starts lagging again. Really the best solution is to just use a separate chat program and only watch the video popped out or from the dashboard. If it works on my seriously old computer, it will work on yours, trust me.