Even if your dungeon takes longer you are instant queuing vs waiting 20 mins as a DPS, then running a dungeon in 15-20 mins. You also have to think you are netting all the loot in that dungeon on top of the class specific item given to you for beating it, which in turn can be used for alts classes you level later or turned in / sold.

If you are a competitionist who likes to max systems then you probably are also someone who will max out multiple classes. Not everything needs to be maxed out within a month.