Some are ahead (more precisely, they move at the same speed), some are not ahead (for example, with a flagellum defect). But we will never know about those who have lagged behind: they will not fertilize an egg and their defective genes will not be passed on to the next generation. And we see those defects that affect not the mobility of sperm, but something else.
Because mutations (including malignant ones) of individual genes have almost no effect on sperm motility and their ability to fertilize an egg. They have an effect only later on the formed organism.