I heard about the lag but we ran a 25 man Naxx raid last night (also EU servers) and it was fine. This could be the first instance in recorded history where Argent Dawn has less lag than other servers (!)
 

Imo it's a problem with aoe damage/multiple mob situations. Maybe caused by the new combat log (more data from server to clients) and a lot of players raiding the same instances... They must fix this as soon as possible.

- Ben
 

We had this issue on our server since the previous patch was introduced. Granted, it was attributed to the Wintergrasp issue but it seems there is more to it. The result was visiting each raid instance to see which is doable and play with time. It seems there are days and times where he lag becomes less. The deeper in the content the better.

I feel the fact that Blizz made the instances faster draw more people in them and things got a bit out of hand. As one previous poster mentions, it seems that certain aoe spells (like the dk's), haste calculations and certain buffs/auras make the servers work overtime. Its an issue that could not be investigated into the PTRs or the beta due to the population of players there.

Blizzard seems to be trying to find ways to overcome this but I fear some more fundamental changes will be needed.

Also, Blizzards policy on transfers ought to be more solid to stabilize populations and spread. In the EU servers there have been somewhat questionable choices. Silvermoon for example had all the Hordes allowed to leave for other servers - horde is/was an extremely small minority. More or less the opposite in Magtheridon. At some point they allowed hordes from Mag to come to Silvemoon (not sure if this is allowed anymore). Why clear the horde population to try to bring it back a few months l8r?

Couple that with the recent reports of bugs in the last patch (which to be perfectly honest I have not yet encounter in my realm), Blizz seems to be flaky in certain management aspects.
 

Meanwhile still no Wintergrasp :(
 

>> Heroic lag for added difficulty?

Hahahaha, classic comment!

On EU Twilights hammer, the lag was awful... People were even being disconnected from server when we fought certain bosses. Seems to be random whether you get disconnected or not, and it affected certain people more than others. One night will be fine, the next night disconnected on every boss fight. Addons disabled when the problem started, so it's a pure blizzard problem. :)
 

I think that the lag is more widespread than just instances. I've tried to make mental notes about where lag happens. Dalaran is of course very bad which is because there are so many players there. Then to me it seems like phased areas seem to be somewhat more lagged than other places, and of course then also if they are more populated than other areas. Fjorn's anvil seems to be a place where I lag more than other places. It's still just random observations from me but it could very well be like that.
 

No, not heroic lag. It's called *epic* lag :))
 

Your whole battlegroup appears to share the hardware for Naxx 25. So its not the 1000 players on your server, its the 10000 players from your whole battlegroup.
 

Exactly right. Sharing servers across the battlegroup. I spent 10 minutes the other night with 4 other people running into and out of an instance gate getting a message that there quite literally were no available instances. All of us kept running into the gate trying to snatch up the next available instance slot before anybody else could.
 

Last night the lag was so bad when I was healing heroic VH that my instant cast spells were not so instant. All while the lag thing shows green.

Wintergrap crashed the Northrend server repeatedly on patch day, so it's disabled on mine too.