I tried to simply insert it like any other scenery. It shows up and looks almost correct - BUT the framerate drops about 90% from a 30-40fps to 3

And when I start to move around (if you can call it "move" with 3 fps), the taxiways for example seem to "vanish". Also the (normally) yellow taxi lines are grey...
Any ideas?
EDIT:
as far as I could find out it seems that the scenery is much older than FS2004

I tried some scenery editors like AFX, ADE SCASM and BglAnalyze. Neither of them was able to convert...