We all know that swatting an annoying bloodsucker is not difficult - in most cases, our reaction is enough, and the punishing hand manages to catch the mosquito before it even flies. But to do this trick with a fly is incredibly difficult - you need an excellent reaction.
And it is not so much the speed of insect's flight (though it is much faster and it also plays a role), as the speed of separating from the surface. The reaction to danger in both flies and mosquitoes is at the same level. But the way they take off from the surface is very different.
When a fly notices a danger, it powerfully pushes away from the surface with all its paws, and only then do the wings come into play. A mosquito (unless, of course, its trunk is immersed in the pore, which makes it absolutely defenseless) does not push with its legs, but works exclusively with its wings. And this is understandable: otherwise, when examining the skin area, the mosquito would immediately give itself away, as, for example, the touch of a fly, which, unashamedly, works with its paws to the utmost, is felt at once.
This advantage of the mosquito, on the other hand, is also its disadvantage. Instead of quickly "jumping away" from the danger and then flapping its wings, the mosquito relies only on its wings, which significantly slows down its start.