The claim is that human sperm has different physical characteristics (i.e. phenotype) depending on whether it is carrying an X or Y chromosome.
One theory is that sperm that carry the male chromosome are tiny and fragile but very quick, whereas the female sperm are larger and hardier, but rather sluggish. Source
The male sperm are FAST... but die faster also. The female sperm are slow and steady and have a longer life span. Source
The X sperm (which are the sperm that make females) live longer than the Y sperm (which are the sperm that make males). However, the Y sperm swim faster, so the chances are equally good that you will get a girl or a boy. Source
None of these sources provide references.
Do "male" sperm swim faster than "female" sperm?