Supersampling is a very performance-heavy anti-aliasing technique. Turning this down should increase your performance in pretty much every scenario.
But, I'm not sure why the framerate would drop after the portal. I've only been there a couple of times and was too busy with erm... staying alive.. to really look at the environment. I did notice a lot of shiny high-quality props, but nowhere near as many as in the levels before.
The only thing I can think of, is that the main area is fairly big without having doors. A while ago, Madoc introduced performance improvements using doors as occluders to separate areas for rendering. Having such a "big" area with lots of lights and objects might cause problems on older hardware with your graphics set too high.
But really, this shouldn't give big issues. What's your hardware?