Dynamic lights not appearing at landing it's a known P3D bug, LM is looking into it right now, because KORD obviously use the correct method suggest by LM to prevent this ( using an effect controller), which seems still have issues.
It's not possible the scenery could possibly have any effect on the airplane and nobody reported such bad fps in more than a year the scenery came out.
You said the fps passed from 20 to 2, as if 20 was an indication of "good" fps. I'd say 20 fps is already quite bad so maybe your settings are already too high for your hardware and, landing on a detailed airport might have exhausted your VRAM ( you we lucky it didn't crash ). Next time, check your VRAM usage using the fps counter, and see if approaches more than 90% of the available one. If it does, lower your graphic settings, because they are too much for your card.
