With all the recent defiler of the just runs, I believe I have figured out a bit what causes the issues in mabar, deathwyrm, fire on thunder peak and defiler and so on.

The majority of script-trigger-spawn monsters, fire up an algorithm of what to attack. Monsters that are already static do not unless something happens that causes them to investigate aggro. So, when mabar gargoyles, deathwyrm skeletons, abishai executioners appear en masse, they all fire up 'find something to attack'. They way they choose is broken, thus you used to see executioners spawn in south east corner and decide to attack someone piking northeast without ever having met or seen.

The easiest place to notice this is arena challenges in eveningstar. With the 15% xp bonus going on, we went to run them. We had one person stand in front of each spawn point, 2 in the middle and one was piking at the entrance of the challenge. On entry, almost all the mobs would RUN ignoring everyone else to the guy piking at the entrance!

Now, when this happens in big numbers - aka 20 players in a mabar instance and 40 gargoyles trying to decide whom to attack - you get upload lag. The servers give you information correctly but they don't accept information back. In other words, the server knows the monster is attacking your toon but is ignoring whatever your computer is telling him your toon is doing because he's too busy trying to decide which character to attack.

I believe that for the majority of the encounters, a simple 'attack the first thing that comes NEAR you, then IF something damages/aggroes you START doing intelligent calculations on which warlock to attack first' would fix a big chunk of it.

I'm pretty sure the majority of the TOEE lag on release was also related to this as the monsters usually script-trigger-spawn. When I ddoor and run back through, I can see them appearing out of thin air in front of me. I bet having two-three suchs triggers spawn mobs at once would cause the exact same thing.

As an afterthought, whoever thought leashing the executioners in dotj would fix lag INSTEAD of having them not randomly aggro 1 in 12 of the party irrelevant of location (I know thats not the exact maths) is not very smart.