Just to add what others have said, the framerate drop seems to be related to what the aircraft is pointed at. I took off from Vazani in the SE direction in a custom mission and it was smooth as silk coated in butter.
I kept a rough SE heading for a while for a while to get a feel, but when I turned back in to face towards the airfield (not in visual range by this point) the FPS plummeted to completely unplayable rates. I'm talking slideshow rates. My alt was around 3400 FT.
When that framerate drop hit, going to third person removed the problem as others have said, but when I went back to cockpit view it turned into a slideshow again.
When I then turned the aircraft SE again, the framerate went back up and it was perfectly smooth again.
I had the flood lights off, in case that matters.