I noticed the NPC count was reduced but the lag still remained somewhat so after removing NPCs individually I found that Cassandra ID 9010010 caused the majority of lag. When this NPC was removed I had no issues with my framerate at all. This may tie into the reports of quests causing this lag and there being an issue with events.
You may have already read the forwarded messages from GM Tonyra and this may have been a part of this unscheduled maintenance, I don't know. Like I said, it's not a guarantee that this works because I am not able to test on every persons account that has a major issue with the Event Hall but I believe this is worth trying. Players can access Cassandras quests via the Star icon on the left side of their screen.
Steps to reproduce: Enter Event Hall (preferably as a high level character)