With regard to the leak suggestion from RF which he has so eminently related too as I would not have known off hand where to find that thread.....
Make sure all areas are sealed if necessary by using an outer boundary of wall segments all around any possible potential leak if not all of your construction. i.e. it may be particularly necessay to do this around known possible offending objects. Corridors, ducts, lifts, curved wall areas and more - enclose them inside an outer room made from simple room segments.
Dont do it unless necessary - first you need to try isolating the reason for any slowdown, but bear it in mind as a weapon that can be used if all else fails. Though as a point of fact I have found that doing it does not add unecessary polygons or add to slowdown as its more likly to help increase fps by sealing off offending objects and restricting what FPSC sees and renders reducing the scene as far as the engine sees it down into smaller pieces and thereby making more efficient use of scene rendering and management at the time of compile and refelecting that at game run time.
In editor test run - to see what may be rendering including world objects and entities within any one scene as you move from area to area use wireframe in editor - to see visually whats rendering unecessarily outside of any immediate scene and take steps to eliminate the numbers of objects if necessary.
Dont use the FPSC statistics reading that you can pull up to view those as they dont matter at all - unless you use your brains and make the decisions about how "You" can force an improvement - the statistics feedback readouts that FPSC supplies wont be of much help and your level wont show any change as you build.
This might be different if FPSC was stable but its not so the statistics it provides are often erratic and illogical. Your own brain is better, much better.
Looking in wireframe - you can see or work out what may be causing a problem. Its that simple - see it - fix it. Too much of anything in view at one time is not needed so dont leave FPSC render or activate stuff outside of the immediate scene which it will often do.
Of course this may not help the original poster as he may have another or multiple issues which could cause a slowdown. Tracking down a cause can be time consuming - In FPSC any single erronous or incompaitble - disliked object/engine issue can cause serious problems affecting gameplay speed badly. Even a single bad line or command in a script can cause serious slowdowns and other problems.
"I am and forever will be your friend"
