Running to and from the quest still takes the same though and so does running within the quest.
Instakilling (assuming proper ability) also takes the same time and some mobs you simply overkill on R1 meaning they dont really take longer on R4 or not as much longer as the dps numbers would indicate.
So, its not as clear cut as measuring dps as thats only part of the equation. Some quests you can probably run using only 10% more time on R4 compared to R1 everything included even if the dps indicates that it should take more than twice as long, so its highly quest and group dependent.
As an example from some xp/min numbers I collected a while back - Ghost of Perdition took me roughly 8 mins on R1 grouped included getting to the quest and roughly 9 mins grouped on R3 so R3 is better rxp/min but worse xp/min for me in this case. This wasnt a controlled experiment but still shows concept.