Yes but thats the catch, because we all know how rediculously CPU intensive FSX is. If the coatl engine is now taking more processing power from my CPU to run all the airport services, it is definetly going to have varrying degrees of performance hit if it is not yet fully optimized.
That's exactly the opposite: if FSX has a problem, is that it doesn't use ENOUGH CPU! Of course, I'm referring to the total amount of a modern multicore system. The biggest problem in FSX is that it occupies ONE CPU at 100%, and the rest not really so much.
Which means, there's a lot of spare power in those other cores, enough probably to run another entire simulation in parallel...
If Couatl was an internal module, running in the same process space as FSX, it would have to either not being multithreaded at all, so it would have the ability to decrease FSX fps or even pausing it when doing its own calculations, or if it was multithreaded, it would have real concurrency problems working with FSX, which is not really thread safe, having a multithreaded module inside FSX, assuming it's even possible, is so complex and unreliable, that I doubt any FSX developer has even tried it.
Which is why, OTHER modules, loaded as .DLL, have the ability to slow down FSX, but NOT GSX.
BECAUSE, if you followed all the explanation so far, with Couatl being an separate Executable, has its own separate threads that can't conflict with FSX, so it can be as multithread as it could without any issues AND, it will be scheduled automatically by Windows to use those famous spare CPU cycles that every modern systems has in quantity.
So, to give an example, if GSX needed to calculate a path for a vehicle on a very complex AFCAD with hundreds of nodes, the only effect of this would be that the vehicle might start a *bit* later, but FSX wouldn't be slowed a bit in that time.
Not to mention that, when you are not using GSX, is not running any code at all, let alone a complex one (which wouldn't slow down FSX in any case).
That's why I'm saying it's really not possible that GSX could slow down FSX, just by being installed.
And, if you already had any FSDT scenery before, you already had Couatl running in the background.