I never thought I'd be asking a question that sounds as absurd as this, but I think my internet connection is rendering Diablo 3 singleplayer unplayable for me at this point. I have a 1.5 mbps connection with a ping of about 55. During play sessions, I will randomly lose the ability to execute attacks. My demon hunter does the animations, but the attacks themselves don't activate for a few seconds. Traveling into new areas also causes a big lag. Things started to get really obnoxious when I watched the cutscene of my character finding Deckard Cain. Even though the cutscene should have ended, it stayed up and the characters did an idle animation for about 2 minutes. After that, I was greeted with a death screen because while I could only see the cutscene, my character was actually being slaughtered.
Anyways, is my internet connection too slow to communicate with the Blizzard servers and causing all of this lag? I know it's not my actual PC, considering I'm running an i7 2600k and a GTX 570. I also have never had problems playing online games (even Starcraft 2) with this internet connection before.