When the hard-working little swimmers encounter the thicker vaginal mucus, their path is slowed. So the sperm often join together at their heads, which gives them greater swimming speed (up to 50 percent faster) than if they were to carry on individually.

I wonder why that is. If a group of people were to join together and run, the speed of the group would be capped by the slowest runner. And aerodynamics would be worse.