First let me start with system specs:
3570k @ 4.4Ghz
16GB DDR3 2400Mhz
GTX Titan 6GB
120GB Corsair SSD where the game lives
My performance varies between smooth (60-120fps) and awful (<25fps) and always when looking at buildings or being in them. This is irrelevant of settings as I was running everything on stock settings at the time and bumped them up later with no change in performance.
I'm fairly competent with the A2/OA tweaks so I've done everything from gpu max frames ahead to the w buffer, changing field of view, everything under the sun.