Surely not a problem of the scenery. 4.9 fps it's really impossible on that system, so it's surely a problem of your video card settings and/or drivers.
And, it it's not very clear what do you mean with "all fsdt airports run great except LAX". Which update you are referring to ? the KLAX last update ?
And, does it mean that *before* the update, the other FSDT airports ran bad, and now they all run great (except KLAX), or they always ran great except KLAX ? Or that KLAX *used* to run great, just like the other ones, but now (after the update) it's the only one that doesn't run great ?
The update itself can't have any visible effect on the frame rate, nothing really important has been changed and (of course) nobody else has reported such a drop in frame rate after it, so it must be something else you changed in your system. Or maybe just because you install it from scratch, you forgot to install something that was installed on the other system, like any kind system/video/audio drivers or directx or anything else.