Sperm is highly competitive.
From Science Daily: When it comes to mating and determining whose sperm reaches the elusive egg, females control both the playing field and the rules of the game .
Even the length of sperm makes a difference. “Females choose among males based on the length of their sperm. Long sperm tails are thus the post-copulatory, cellular equivalent of long peacock tail feathers.”
Research also shows that speed of sperm matters too: see Primate sperm competition: speed matters.