I downloaded a couple afcad files for airports that I don't fly to often but fly over a lot.  One of them is LFPO by Ray Smith.

 

When I fly over the airport at cruising altitude, and look down from the virtual cockpit, my fps goes to single digits. It is only a default airport. Is the object file some how slowing everything down? It does not make sense why it would be worse than a payware addon airport. There are only default airports nearby no payware scenery.

My guess is that it is a combination of the airport and its proximity to Paris.  I expect it is just a scenery intense area and adding a much more detailed LFPO is really taxing your computer.