Some interesting comparisons here, KLAX against Zurich, FSX vs FS9, zero AI and default airplane, which is the only sane way to compare sceneries.
First is KLAX, an high polygon scenery with no FS9 code in FSX, and it runs faster in FSX than FS9!, polygonal complexity is obviously the same (the models are the same) the texture folder in the FS9 version is 170MB, the FSX version is 270MB, and the FS9 version doesn't have animated ground vehicles, bump mapping, elevated taxiways, and jetways are static, but FSX is faster.
Then we have Zurich, way less polygons than KLAX, which under FSX is slower than KLAX (it still has lots of FS9 code that FSX doesn't like much), but is really fast under FS9.
This simply confirms what I was saying all along, when you cross a certain threshold of polygonal complexity, FS9 just can't keep it up with it anymore, and of course it doesn't get better if, on *top* of having a polygonally complex scenery, you have a polygonally complex airplane and add many AI models, the overall density becomes too much for the FS9 graphic engine to handle.
It's not that KLAX is not "made for FS9", it's code IS 100% native for FS9, it's simply an exceptionally detailed FS9 scenery...
By *removing* all the FS9 code from FSX, we finally got free to use it as it was intended, and added a lot of detail to the models, which doesn't seem to be a problem for FSX to handle. Before, when we still used FS9 code inside FSX, we had to *restrain* ourselves a lot with polygonal detail, to the effect the FSX version didn't run *that* better in any case, but the FS9 version was really fast.
I'm sorry, but other than giving you the possible reason (which of course doesn't explain the 7 or 10 fps figure someone cited, with systems way faster than mine...) why KLAX could be very demanding on FS9, there's not much else we can say, other than TRY THE TRIAL FIRST.