So last night we got 25 people together for a guild raid, and did our second Naxxramas heroic run. Cleared out another wing, spiders this time, everything fine. But lag was really bothersome. Previously, on 10-man runs, we heard a lot of people from the 25-man runs complaining about lag, but in the 10-man raids there barely was any. Now, our own 25-man raid was a horrible lagfest. I press a button, nothing happens for 2 seconds, I see my spell animation, nothing happens for 2 seconds, and finally my spell lands. And as we just discussed, having your healing spell land 5 seconds too late is a killer. So instead of trying another wing, we went to Sartharion instead, which was less laggy.
I hear Blizzard is blaming Wintergrasp for the lag, but that can't be it. Since the patch Wintergrasp battles are disabled on our server. So I think the problem is that, on that particular evening, there were over 300 Horde players in Naxxramas, and as there are pretty much exactly twice as many Alliance players than Horde on my server, probably over 600 of them, for a total of nearly 1,000 players in a single instance. Who would have thought raiding would ever be so popular? :) Obviously not the guys who designed Blizzard's raid dungeon server architecture. :( Not sure though why the 25-man version lags so much worse than the 10-man. Heroic lag for added difficulty?