Every segment, entity, and even a marker as polygons. They all exist in the lvl, that means they are not activated. Dromant polys exist in a state of mind that is called by some people as: "stand-by polys" Only cretain, but vital functions are active during this time. Until the point in time where you have faced them, they become fully awake. In other words, if you can see them, they are fully active visa-versa if you can't. The reason your frame rate droped so imensly, is beacuse during that scene, you must have been facing less ploys than before; when you are facing the lvl, you are facing more polys. You don't have to see them for them to become active. You just need to face them. Where ever you read that, they were lying to you. I'd try to re-position your ventduct so that it won't ever face the lvl. You could also make a smaller lvl if you wanted to invest your time in that.
I hope this helps some!
Best of Luck,
G4fan
My first game: "Wars of Crime"-31% completed