i have recently noticed massive fps drops in my game, couldn't figure out why, but then i noticed that it is completly related to crouching, and i confirmed my theory with my friend, the same occurs with him.
Basically i lose about 10 fps only by walking while crouching, but the shit only goes ugly when i sprint while crouching, my fps drops from about 65 to 30, really weird, i think lot of people haven't noticed that, but i definetly recommend you guys to give it a look.