is it really lag or just a low framerate?
if it\'s the framerate, try using Adaptive distance (options - graphics - details)
if it\'s your connection, try to reconnect to your ISP - sometimes you get a \"bad wire\" with old copper cables (analog, ISDN, DSL) and chances are high that you\'re better of after a redial
edit: for the FPS problem it\'s also a solution to reduce the screen resolution via pssetup