They decide to calculate how quickly a vampire can drain a person's blood and submit it as a paper.

In this investigation we found that it takes 6.4 minutes to drain 0.75 litres of blood from the human body, this seems fairly reasonable considering it takes less than an hour to give 0.47 litres of blood when you donate from a vein [8]. However this blood is coming from your arm and the blood pressure is lower here whereas ours is coming from the external carotid artery. To take it one step further we could take into account more than 15% of the blood being lost from the body and also the pressure if the vampire was sucking as well as drinking. This would reduce the time taken and make the process more efficient.

They really only calculated how long it would take to drain 15% of a person's blood. After that, heart rate changes and their assumptions about flow rate would no longer hold. So, what? A good 20 minutes to really drain a person? Who has that kind of time? Vampires need to feed and run.