It's possible the fps drop after a while is caused by the terminal interior filling up with passengers, since it takes a few minutes for all of them to appear. You can test if this is the case, by disabling them from the GSX Setting menu, the "Airport walkers" option.
Also, the fps target of 120 fps is probably wrong, just set it to unlimited in the sim and, eventually, limit the fps externally using the nVidia Control Panel.