Is this affecting everyone? I can't imagine hardcore being playable like this anything past normal mode, and in my case, that means Diablo 3 isn't playable... Since softcore is a royal waste of time.
I've found a solution *sort of*
The serious lag, for me at least, occurs during the first 10 or so minutes of a game. After that, it will stabilize at around 200ms, which albeit not optimal for HC, is manageable enough. So a workaround for this problem in the short term is to make your game, go grab a snack or alt tab for 10 minutes, then come back and you should be good to go at barely green ping.