Is there a specific reason? As far as I know, the only difference between Chill and Okay is the loss of "Super-meter" when you get hit.

Both difficulties are pretty much the same run wise, the majority of the runners just grinded Okay way more, thats all.

I heard someone say the AI is more aggressive in Okay so you don't have to chase them as much, resulting in faster times. Unsure if that's true.

The spawn rate and aggressiveness of the AI is slightly increased as you bump up the difficulty. You don't lose as much time waiting around on OK or Gnarly.