A community manager post on the Diablo III forums says Blizzard believes it has cleared up the "major lag" reported in Diablo III earlier this week. Apparently this was the result of the recent version 2.1.2 patch for the action/RPG sequel:

Hey everyone! We believe the latency issues many of you have experienced since the release of patch 2.1.2 should now be resolved. However, if you continue to encounter problems with high ping or in-game lag, please let us know.

Thanks so much for your helpful posts!