Just to confirm... this is in MSFS not P3D.
Ok, I'm sorry, I just didn't noticed which section we were in, because I know KCLT in P3D could use some optimizations, because it came out the day after P3D 4.0 was released, so it was an hybrid between FSX and P3D code, with lots of complex handling done by GSX/Couatl to make out the differences between the two sim, trying to use 64 bit larger memory, without losing compatibility with FSX, which was still used by many people in 2017.
In MSFS, it's even likely to be caused by the airport, which one of the very few airports currently in the market that use multiple LOD levels *extensively* ( it didn't had any in P3D ) and, comparing to the P3D version, it doesn't even have any of our specific features like Docking systems, which were added by GSX so, it should have a much better fps in MSFS than in P3D, both because is more optimized, but also because it has less features, which will came back with GSX.
It is BY FAR our best optimized scenery, and one of the most fps-optimized scenery out there for MSFS. If and when Asobo/MS will enforce the LOD restrictions they gave developers some months to adapt first, KCLT will be the only scenery you'll be able to use without requiring any updates, since it's already done following the best practice for fps and memory optimization.
Also in this case, the last scenery update happened in March so, if your flight in June didn't shows any issues, it clearly can't be caused by the scenery either.