Almost every single dungeon has packs of mobs that can be avoided in the process of clearing them, as they're not in the way of getting to bosses. When leveling, do you clear every single pack of mobs for XP, or skip the optional ones so you can get the dungeon done faster and get back to solo leveling/another dungeon run/etc.? Let's assume for the sake of argument that every single boss in the dungeon is being downed, and you aren't skipping any (ala Halls of Stone).
For example, I was just in a Gundrak run where the group jumped down into the water after we killed the Colossus, then came back up the ramp and killed all the groups of mobs 'for extra XP' before running back down to do Moo'rabi. I had never seen anyone EVER kill those mobs before. It's always been 'finish the dungeon quicker so you can get back in there faster'.