Guide to Fix FPS Drop
MW5 has some wonky engine issue with AF. I know that in every other game since 3D acceleration was a thing, AF is basically an auto-maxed setting, but it really hurts performance in MW5, especially when you have the zoom window up.
Try reducing it to 4x or even 2x. I'm sure there's some small effect on grass textures a kilometer from you or something that you might notice, but I never really perceived the difference... and the FPS gain made any such sacrifice worth it anyway.