The missionserver, as its called, is a separate piece of software that stores the mission data as created by players. It does use some resources, but that part is not a big deal. Once you start an AE mission, it's loaded in a mapserver, like any other mission. They are not treated any differently.
The problem is that players can build missions that are a lot more resource-intensive than normal missions from the game. When enemies spawn in normal missions, most of them have their AI in an "idle" state until they are aggroed, which doesn't require much processing power. But most AE farms rely heavily on ambushes or patrols, which spawn the critters with fully active AI. And that's the problem.
Missions are run on 9 servers with a total of 224 cores. For the most part you can run lots of zones and missions on a single core without a problem; but AE farms tend to max out a core all for themselves. If there's just 100 AE AI-intensive farms running, that eats half of the mapserver processing power for all 4 shards.
Fascinating.
Do we have any info on how NCSoft handled this? I can't believe they were just throwing money at the problem over and over instead of figuring out a way of making the AI requirements from AE missions draw on elastic resources...