If you are paired with BLM or SMN and they are doing their AOE rotations, please attack the mob with the highest HP, not with the lowest!
For example a tank pulls 6 mobs, a MNK single-target them and a BLM AOEs them. Monk can kill 1 or 2 mobs faster, but the whole mob pack still dies as fast as BLM can AOE them all! Monk's DPS was a waste of time really, he did zero contribution to finish the whole pack any second faster. All he did is a just few seconds of an easier time for a tank/healer.
And when a melee finish off the mob which still has 15 seconds of SMN dots ticking, they are effectively wasting SMN's dps. Just leave alone that mob, it'll die from bios/miasma in a few seconds, better switch to the mob which will take more time to die.
A good melee will target swap to burn all mobs fairly evenly, greatly contributing to the speed of the run.