I've seen the frame rate drop, but I get different results on a separate box. The computer I initially installed on isn't super high powered, but as soon as I got airborne I'd see framerate drop down to 6-7, regardless of location. Stable flight would stabilize frame rates somewhat higher, but maneuvering would drop them again.
Installing on my other computer which is much more recent and with an SSD, I see excellent and consistent frame rates. Sometimes I see maybe 2-3 fps drop but otherwise no problems.