exact same problem for me!

I got

AMD Phenom II 6core @ 2.8 ghz
16gb RAM
geforce gtx 560 1gb Video card

i can play Single player with no lag what so ever, max settings getting down to about 25ish fps when bombs explode etc,

then i go into a multiplayer server.. (the ammount of people does no differense), on max settings, about 12-13fps. on the lowest possible settings with about 1500 range i get 15-20 (20 if i am lucky)..

It makes the game unplayable, after 10-15 minutes u get a headace because of all the lags..

hope this is high prio for the developers to fix!