Another thing that matters is which side of the plate a batter hits from. Lefties have the advantage of being two steps closer to first, and the momentum of their swings take them that way anyhow. Their weight is on their front foot, so it's just a matter of pushing off and crossing over with their back foot, using the momentum they had going that direction already, and they are almost up to speed. Ichiro does it best because he is practially out of the box at contact, which helps him get down there faster. He hits more like a slap hitter in fast pitch softball -- put it in play and beat the throw to first. Righties' swings take them away from first, and with the weight on their front foot going that way too, their back leg has to help them reverse their momentum and start the first explosive move in the other direction, which probably costs them a third step, too. Kind of why I'm teaching my kids to hit lefty -- when you are as slow as I am (and they might be if they take after me) the extra steps help.
I think the difference between ballplayers and runners is that first 90 feet. Because a lot of ballplayers have more explosion, they would get to first faster. But if you are racing to the right-field foul pole (about 110M), a distance runner would probably take it because their finishing speed might be better, and they would carry their acceleration farther.
That's really a lot of what it is, how good you are at accelerating. I was pretty slow getting down the line, but I got a lot faster when it came to running extra bases because my opening acceleration wasn't good, but I was still picking it up as I rounded first.
And yeah, don't discount adrenaline. First you get a shot of it just standing up there, and you are a lot faster getting down the line when you are the potential last out of the inning or whatever.