Sarah had asked me if I could find any cause for why cassandra would generate massive amounts of lag for the player. I looked in the WZ files and could find a reasonable cause.
Inside the data for Cassandra it contains what all other NPC data contains - a stand animation and perhaps a few quest-specific animations. However, Cassandra is different from other NPCs. Her stand animation changes depending upon what the current date is. So, every time when her stand animation is loaded the entire file must be read (which is quite large for an NPC - larger than most NPCs) and the current date must be checked against all of the dates given for her conditional stand animations, and then it chooses/displays an animation. This would be likely to happen every time one of Cassandra's animations finishes or, if their programming is absolutely terrible, every time an image is loaded.
I can see how this would cause lag, especially on not so up-to-date computers. So for those who are experiencing lag around Cassandra, please tell Nexon to remove the conditional animations and to change her typical stand/eye animations only when it's necessary and not based upon a date system.
EDIT: It looks like Gaga suffers from the same conditional animations as well. He should be fixed.