The scenery doesn't have a very big impact on fps per se.
If you are seeing low fps on FS9, surely you must have it loaded with other things. The number one culprit is usually AI traffic, if you are using an addon product with a busy schedule, the number of AI planes in the area (don't forget, there are also other airports around, like KMDW) might easily exceed 250-300, at 100%. If you are not using fps friendly models, like WOAI, etc, it's just too much to handle and even if you are using it, the fps impact is noticeable.
Check how the fps changes with each AI density (start from 0 then go up to 100 in steps, and see what's happening).
Also, you can try lowering the scenery complexity a bit.
With a video card with less than 512MB, you might also try lowering the maximum Global texture size one notch, it will decrease a lot the vram requirements. Note that, since the FS9 version of scenery is using the same texture size of the FSX version, with reagards to vram requirements, the two sceneries are basically the same, so you wouldn't be able to use it an older video card "just" because you are in FS9...
There's no free lunch, the scenery is already heavily optimized, just compare it (if you have it ) the fps with the old Simflyers scenery and see the difference.
