Barring trap difficulties, getting an experienced full group of 6 shooters through a parcour in 35-45 minutes is not a problem. Even with a first-time shooter or two, the vast majority of the delays come from trap issues. One broken trap can set a group of 24 shooters (old style FITASC, the only way to play the game) back as long as it takes to fix the trap.
The absolute best way to deal with this is to schedule at least 50, and preferably 60, minutes for each parcour. This gives shooters, referees, trap repair folks, everyone a bit of flex time after each rotation to accommodate all the thousands of things that can delay an outdoor event that relies on complex mechanical devices. I understand that this sometimes can't happen, but in those circumstances extra care has to be taken to make sure that the traps are uber-reliable, and easily and safely accessible for repair/filling as needed.
If the details are taken care of by the shoot management beforehand, and enough time is scheduled to move between pegs and parcours without rushing, the requirement to speed up the shooter from 20 to 15 seconds becomes meaningless. The rule change seems to move the responsibility to keep on schedule from the shoot management to the shooter, which - to me - is the wrong way to go.
I never, ever start a group of shooters early because someone is inevitably running "just-on-time." I have very little patience for shooters who show up late, and fortunately neither do the rules.