There is something very strange I saw in this last contest "Rayan"

Why many LGMs took time like 10-20 mins to solve div2D (this time is equivalent to 2 or 3 hrs of me or maybe more :) ), when I saw that and it was the end of the contest I stopped thinking automatically (not letteraly, like with no motivation) then suddenly many of my friends some of them are Expert about 1750 solved it in nearly this average time and another friend who is ~1150 solved it in nearly less than hr (but unofficial)

is this because constructive problems normally take time or what?