I know you have a video showing everything is just fine but that is with a default aircraft. I have done the same thing with a default Piper Cub and got the results you did Umberto. I am sorry to say you do not optimize your scenery's for 3rd party aircraft like FSLabs or PMDG or addons like ASP3. It seems you only optimize for default.
We make the scenery as best as we can and, if it runs well ( as you noticed ) with a default aircraft, it means the scenery is ok. That doesn't obviously mean there isn't always a better way to do things.
KCLT is still a scenery that support FSX, P3D1, 2, 3, 4 and 5, and this without ever asking any extra money for updates, so those that bought it for, let's say, FSX when it came out, could continue using it with P3D V4 and they can continue using it in P3D V5.
It has *some* optimizations for P3DV4, but not nearly as much as a true native P3D4 scenery like KORD V2 or LFSB, which are 100% PBR and 100% made for P3D V4. There's no such thing as a native P3DV5 scenery, the format of 3d objects exported with the V5 SDK is still 4.4, the only difference is the AFCAD if the scenery uses the new Sloped runway feature, but it's otherwise identical.
Because KCLT had to support FSX too, it had to rely a lot on Simconnect to create objects, because many things that can be done with the V4 SDK with the standard SDK, could only be made using Simconnect before. And, there was no way this airport would fit in memory with FSX without a strict memory management, which is done by creating objects only when they are needed ( when you are close ). This is done with Simconnect commands.
The problem is, if you have OTHER add-on that send a lot of commands through Simconnect ( weather add-ons are an example ), it's possible the sim cannot keep up with all the requests, so the various Simconnect commands we send to the sim to create/destroy objects, which is usually done to keep fps up and prevent memory exhaustion in 32 bit sims, might be executed more slowly, or the sim can pause briefly, because it's busy talking with everybody else too.
However, thanks to the flexibility of our Couatl scripting engine, it's fairly easy for us to tweak the memory handling strategy without having to redo the whole scenery. KCLT already behaves differently if it runs with a 64 bit simulator but, those optimizations weren't enabled for the whole scenery, just the main buildings.
We did the same with KSDF when a user reported the same issue ( KSDF is similar to KCLT, it's very detailed but still support FSX ), and with a small update, it improved the stuttering a lot.
If you run the FSDT Live Update now, you'll see a new update that does the same for KCLT, and you can even disable/enable it using the YouControl menu for KCLT, it's a new option called "Extended loading range", which by default is Enabled, if the simulator is 64 bit.
Yes, there IS a cost in fps, nothing is free in this world, there was a reason why we kept the loading ranges fairly short, and was both to save memory for FSX users, but also to keep the fps up. I've measured the fps loss with the PMDG 737 NGXu, with AI traffic, at about 2 fps, but I couldn't see much stuttering.