Packed being the key word here. If there are already 3 parties in the early 30s in the citadel, the mob repop rate doesn't really support an additional party, so then you have to wait until someone in the existing parties leaves, and hope that you're next in line. And then when you finally get in, the party might just disband after a 20-30 minutes anyway cause all the others have played for a long time and are getting tired too. And then you're in a situation where few people who are left say something like "well let's just head back to jeuno and regroup" and all that is easily a whole hour passed with a total of 15 minutes of actual killing.