I noticed that the world record holder has a faster start than i. Is there some things to improve to get a perfect start ? How are the pros starting that quick ?

Clutch kicking, and starting in 2nd are faster on most tracks. If you find a run with a controller overlay you'll see a button (B for me, Y for most) being pressed repeatedly. That's the clutch. I'm not great at it, but there's a rythm to getting fast starts by doing it.

thanks, that was exactly what i was looking for :)