I don't know how relevant this is, or if it's specific to me, but any time I'm fighting a sandstorm and it opens up with full frontal weaponry, it slows my framerate down significantly. It's the only time I ever experience slowdowns in game, period. I don't know if it's an effect thing or what.
I'm on an i7 with 16gb of RAM, an SSD, and 2x nVidia 755 in SLI.