Do male sperm cells swim faster?

Studies show that the sperm that produce female offspring (aka girl babies) swim more slowly than the sperm that produce male offspring, but they are also hardier.

This means that "boy sperm" swim fast but die faster and "girl sperm" swim more slowly but last longer.

The "Shettles Method" says that to increase your chances of having a girl, have sex 3 days before you ovulate and to have a boy have sex the day before you ovulate. The theory being that if you have sex earlier, only the "girl sperm" will survive OR if you have sex later, the "boy sperm" will outpace the "girl sperm."