Hrmmmm...
On my system it seems that any portion of the sky will hit the framerate pretty hard, unless it's through UZO or Binoculars. I don't like using persicope view unless absolutely necessary. It turns the simulation into a fast slideshow. In Freeview I get bad framerate until I rotate the camera enough to get the sky out of view.
I don't know how they render the sky in this sim, but I get the feeling that it has something to do with the framerate hits I'm taking in periscope and Free views. Underwater views are fantastic, though.
I have an onboard Mobile Intel 945 Express chipset (major part of the problem). Everything else on this machine is up to spec -- plenty of RAM and whatnot.