I’ve watched the VOSA video on daily checks and at no time does anybody say it should take 15 minutes, in fact they don’t give a time, but just show you what to check, do this in 5 minutes or 25 minutes and its ok, very much depends how well you know your truck, there are things like calibration dates and trailer MOT that if they had 6 months to run when you checked at the start of the week will not really need checking for the rest of the week.
However if you are told by some micro managing logistics operation to take 15 minutes then that’s what you do. I personally think the 15 minutes comes from the various timing exercises on the operators cpc exams, as it makes the calculations nice and easy.
But as Newmercman said regardless of time taken if you miss the blindingly obvious the DVSA chap will suggest you haven’t done you checks properly regardless of what time your tacho shows as other work at the start of the day.