Before someone asks what ATB is: in ATB battle, you have to watch a gauge load. Only when the gauge has loaded completely you can attack. Speed affects how quickly the gauge loads, which means faster Pokemon will be able to attack two or four times before a slower Pokemon gets one attack in.

https://www.youtube.com/watch?v=LG5hr18qUVg

See the time in bottom right corner? Only when it has filled up completely you can attack. Mods this is not a troll thread, I legit want serious discussion