I finally own Crysis 3 MP and just had a few matches. When not engaging in combat my FPS at:
1920x1080, Fullscreen
Everything Medium.
No AA
No Motion Blur
Anisotropic Filtering set to 1x
Lens Flares enabled.
HD5870 with 13.2 Beta5 drivers.

my FPS dips to less than 30. No problem when not engaging an enemy, but like I said, fire at an enemy, is it the water shading/physics or the armor particles, my FPS dips down to less than 30 and I die, cause due to the lag at LESS THAN 30FPS I was unable to properly aim at players.