fps on runway: ~20
fps ~1 mile aout of KLAX: 30 (limited by external controller)
fps on close approach to KLAX scenery: ~5 and heavy stuttering
Daytime, no AI traffic, no aerosoft Las Vegas, KLAX v2 downloaded 28.6.2013
5 fps with default Cessna and no AI, it's not normal, at all. It's nothing related to the scenery, most likely it's your video driver settings, and it happens only at KLAX because it's a scenery that relies heavily on the video card, so it requires the video driver and settings to be correct, while older unoptimized sceneries with lots of FS9 code, relies so much on the CPU, that you will not notice a problem with the video card settings there.
-Would a reduction to 2048x2048 bmps boost fps and if yes: how can it be done?
You can try that, there's a setting in the Addon Manager that does just that. If you see an improvement, it means you are exhausting your available VRAM when at 4096, but I find difficult to believe you are, with the default Cessna and no AI.
Do you use the ENB Series program maybe ? If yes, you might have to turn if off, because I do recall a drastic effect (like half fps with it) on fps at KLAX when I've tried it. It's logical because, the ENB program needs lots of spare GPU power (it's designed to enhance programs that don't use the GPU much), but you don't have that much spare GPU power left on a scenery that uses the GPU a lot.