Now by performance I don't think they mean "My pocket Paladin can't tank."
What they're looking for are detailed reports such as "When I was attempting to do the Heigan dance, my entire group lagged behind a good 5 to 10 seconds. We decided to go and raid Hogger after we wiped for six hours in a row." Or "The Ice Block appeared after Frost Breath when my guild was taking down Sapphiron because of the 30 second lag."
Matt Rossi wrote a great piece yesterday on the need for designing the fights around latency. Hopefully this will become less of an issue as Blizzard works out their backend mechanics; and it appears this change is a major step forward.