I have the same problem in Single Player. Not the amount of times that I have it on MP but I have the same issues.
Specs: http://forums.bistudio.com/showthread.php?162894-re-Beyond-recommended-requirements-Terrible-Performance&p=2503003&viewfull=1#post2503003
I know that I don't have a high-end PC but reading so many other people, I don't think It's just because of poorly optimized missions and that kind of stuff. I Know that optimizing an app it's hard, I did it so many times, but man, that explanation just does not fit for me.