It depends on the distribution of times. Over a long period the average person should be able to achieve the mean time. But if the mean is distorted because a few take very liitle time then most of teh time you'll struggle to achieve the average.
So if the mean time for a task is 7 minutes based on the following 10 recorded times:
8, 6, 7, 7, 6, 8, 7, 6, 8, 7 (mean=7) then the mean time of 7 seems fair and should be achievable over a reasonable period.
But if the mean time of 7 minutes is based on the following:
2, 8, 9, 2, 8, 8 ,8 ,8 , 9, 8 then on 80% of occasions you do a task you won't meet the target of 7 minutes- but if you are lucky you may do it in 2 minutes. In this example the median time of 8 minutes may be a fairer target.