OK, each and every large-scale fight in co-op is turning into a latency-fest.
In this evenings 4-person co-op, I took a few screen shots while my mouse hovered above the latency bar, and here is what I have learned:
When we entered a battle with elites (did not notice on lesser mobs), the latency spiked up to over 4k. Each time a group member died (lag will do that, you see) the lag would drop considerably, until we were all dead. When folks began to resurrect at the last waypoint (only a couple steps from the battle) the latency began to increase with each resurrected player. Which brought us right back up to abilities not working, jumping around the screen, and then appearing slaughtered yet again.
In a word: unplayable. Another word: gamebreaking.
Dearest Blizzard, please take this issue seriously. It is all on your end. This is not a random issue on our ends. Our entire group was effected and it very quickly made me quit the evenings play in disgust.