it is not your pc, it is not your internet.. it is something between you and blizzard. I am experiencing same lags. I contacted my friend in other country and for him, there is no problem. So it is happening to only some of us, otherwise there will be much more complaining on this forum. Sad thing is, blizzard is not doing anything to fix it I am afraid..
Today I noticed the network lag is skill dependent. I am playing barbarian with whirlwind, today I played with death blow and the game was fluent, but when I tried to use whirlwind again, the lag was back.