Speed runs are generally done by pulling all mandatory trash up to a boss, AoEing it down, killing the boss, and repeating until you're done. If you have the gear for it, it greatly speeds up dungeon runs. Alternatively you can avoid much of the trash either by simply not aggroing it, leashing it somehow (see the teleporter in Prae), or sac pulling.
In 1.0, there actually were rewards for completing the dungeon under a certain amount of time. These were removed in 2.0.
