In the Add-on Manager it says that GSX is active but not installed. Could that be the reason because it thinks it is on the demo mode or something
Are you sure it says "not installed" ? It's not supposed to say anything other than "active" or "in trial mode". The "installed in the scenery library" or "not installed in the scenery library", is listed only for sceneries that are supposed to be installed in the scenery library.
And of course, it wouldn't make much sense to have the software performing worse when in trial...this doesn't have anything to do with the activation and in any case, KCLT is an airport where GSX is working in full mode, regardless of the activation.
Please try my previous suggestion about lowering the settings, and see if it makes any difference.