See the attached screenshot showing the FPS at A101 at KSDF
See the attached screenshot, at the same gate, same airplane, before and after GSX. The fps impact is there, for sure, but not nearly as high as yours, which clearly indicate something is not right.
What's your settings, especially the anti aliasing ?
You can see that there are three cargo loaders running at the same time in the attached screenshot.
That's because you are trying to use the wrong airplane for a Cargo stand. As explained on the manual, to have the Cargo stand recognized as such, you must use the correct airplane, one that has a forward Main Cargo Door. This will configure the cargo stand as intended, with the loader starting on the platform, and no trains, because the cargo will be moved from the loader to the platform. Basically, the most important feature of the scenery...
747s do not park there, they park in the open space on Ramp 11.
However, it's not that you cannot use a 747 on a Cargo Stand. You obviously can, but the parking will then behave as a normal parking, which will result in all 3 loaders, with the trains. They might be forced to do awkward movements or start from unusual positions, because that parking spot is NOT designed to have all that vehicles: it's made to use the platform instead.
Not that this would have a great impact on the fps because, as you can see in my screenshot, the fps is still normal, however the GSX impact there is be a bit higher than we tested it, because we assumed (hoped) users would only park the proper airplanes there, which will result in less vehicles, and on the open parking space, the fps is surely higher, so having more vehicles there is not a problem.
But again, as you can see in my screenshot, even with a fps-intensive plane as the PMDG 747, parked in the wrong place, which means all the loaders/trains together, I still have a reasonable frame rate, even with my now almost "old" nVidia 980.