The faster that AED gets on, the better chance you have of getting that person while they're still in that shockable rhythm. Once they're outside of that shockable rhythm, your only chance at that point is drugs, and the general public is not carrying epinephrine or vasopressin, or things like that, to try to restart the heart.
Like I said, the amount of time you stay in that frame, it's about four minutes before you start having brain damage kick in. The heart can be gone sooner than that. The faster that AED gets on, the better chance of survival you have. Once you're out of that critical two arrhythmias, that's it, you can't resuscitate them back using AED anymore.