This is a very well known issue, and it's not caused by GSX, of course. It's a COMBINATION of many add-ons all adding their own Simobjects, until you eventually reach some limit, which results in airports disappearing at landing, fps drop and, in the worse cases, the sim crashing.
There isn't a single responsible for it, every add-ons contributes a little to it. In the GSX Manual, in the Troubleshooting section, you'll find several suggestion how to reduce GSX own contribution to the limit, but of course the more effective way is acting on all settings and all add-ons that might insert Simobjects in the sim, like AI traffic, for example.
