The only thing sure, it doesn't have anything to do with our sceneries.
Nothing happens after 60 seconds, all objects are loaded when the airport enters in sight. If anything, you should even have a slight *increase* of performances (more precisely, less stuttering) after a while, because everything has likely been cached in the vram.
One thing to check could be temperatures of both the video card, the cpu and the overall internals. It's possible that, after a while, the PC overheats and clocks down automatically in order to protect itself against burning. That wouldn't be so uncommon, in this season (if you live in the northern hemisphere...)