So little you can barely notice. Hell, I had a tank pull every single group separately in Braflox HM, and I never felt it was taking that much longer than room-wide pulls.
My take on it is that it probably takes even longer because there is so many mobs taking so much space and so many AoEs you spend more time dancing than DPSing, then the healer has to pump out more heals and has no time to even DPS at all...decreasing the overall DPS. Nevertheless, no matter what is more efficient, the difference in time gained or time loss is barely noticeable. Were speaking seconds here, not dozens of minutes. I completely agree with you on every front.