on what graphics are you playing?
anyway seems that processsor is your bottleneck(aka weak place)
but still it shouldnt lag like that
i have amd turion p540, that is 2cores, 2.4ghz, and i can get very stable 60fps(at exesive ammount of players and cars can drop to 40, sometimes it even goes to 100fps, and thats on max graphics, including volumetric shadows and aa x3