The NFL is different than high school and college. Don't ask me why, it just is. Because if it was 10-15 seconds to get subs in and get out of the huddle, then why is there 40 seconds in an NFL playclock? They take longer than just 10-15 seconds on most occasions. Especially Peyton Manning. He doesn't come out of the huddle with 30 seconds on the playclock and just sits there making audibles until 2secs. NFL teams usually get out of the huddle with 20-25 seconds (give or take 5) and that's why Peyton always has 2 seconds to hike the ball. They get to the line with 20 seconds left and he makes his calls and by 2-5 seconds left he hikes it.
Accelerated clock sims the time it takes for subs to come in and out, the play to be called, and the QB to tell the play (NFL plays are very long). The difference between 5 minute quarters with no acc clock and 15 minute quarters with full acc clock is the number of plays you can run. Since the acc clock sims 20 seconds every play, your not picking a play and hiking the ball with 35 seconds on the play clock anymore. The game clock really starts to go by faster than you'd think, but you still get a realistic amount of plays off. 5 minute quarters aren't realistic and you only really get like 30-35 plays for each team. With 15 minute quarters and full acc clock, you get a realistic amount of atleast 55-65 plays for each team.