For trying to conceive, is it true that girl sperm are slower? so to try for a girl have sex a few days before ovulation?

Not exact science: "Sperm carrying Y chromosomes apparently swim faster but do not survive as long as their X-carrying counterparts. So if a couple has intercourse right at the time of ovulation, the faster-swimming male sperm have an advantage, and a male baby is slightly more likely — but only very slightly. And, if the timing is a day or two earlier, more of the male sperm die out, and a female baby is a bit ..."