for internt within your country, 5 mbps and you won't experience lag (well this is anecdotal from narolf). 10-12 mbps and you get very lttle lag with people in another country (I didn't have much lag with a british guy and im in the midwest of the united states) 20+ my highest internet was 18 mbps but i'm assuming with 20+ you dont lag at all unless your opponent has a terribad connection like tkg.
