In the recent offering of instances, the game returned to an old mechanism: In several fights, it is not only not necessary to dispatch the boss as quickly as possible, it is detrimental to the fight.
In a lot of the content we play as we level up, killing the boss “first” or just quickly makes sense. Example: Halls of Night. When you kill Nightmare, his adds all disappear. So, by killing him, you have shortened the battle and reduced the number of dps hits needed to complete the fight (by which I mean, add up the morale value of every add you have to kill for the duration of the fight and that is the damage output your group is required to do).
In some others, such as Silent Street first boss, adds arrive based on the boss’ health level. The faster he is killed, the more the adds stack up. When the boss dies, the adds don’t disappear. You still have the exact same number of damage points to inflict in order to finish the fight because all his minions still have to defeated. In this kind of fight, killing the boss first accomplishes only a false sense of power … it is more elegant to dispatch the adds while keeping the boss “on ice” by not hitting him directly til they are gone. This is why you see the request for “no aoe” in some fights.
If you can spot those fights where speed matters, and where it doesn’t you, you can arrive at a more successful outcome with a real sense of control and cleverness to the play.