That processor is... quite outdated... and as far as I know the system requirements are a damn lie - my brother had an intergrated video card, 700mb ram, 2,8ghz single core processor and the game ran on maximum 40 fps, in cities it was like 10-20 fps. I have no idea why is that, but I have said and will say again that the engine seems to be very unoptimized. Also, you are refering to low frames per second (fps), lag has stuff to do with your internet connection.