In most Live Races you'll have to hit the Launch button as close to "Go!" as you can to get the best start. At the end of the race, you're shown how close you and your opponent were - a positive time means you launched late, while a negative time means you were early

Both of these mean a slower race time - with late starts you're slower on the gas, and with early starts you're penalised with less grip off the line, which affects your acceleration.