OK, so if you're taking a scientific approach, what have you concluded so far? I think most questions have already been brought up.
Did you:
- Check with the other divers to see how they felt about the dive?
- What was their / your air consumption?
- How was your trim? With a new config, perhaps you weren't totally comfortable, causing extra effort
- Your physical condition that day? Maybe that wasn't your day to go diving
Taking a tech diver / dive leader approach, are you going to debrief with the instructor how the dive went and his feedback? As a professional (in terms of having completed the professional level training and acting as a DM on that dive), you should be comfortable doing this with other professionals.
- Your positioning regarding the group
- The instructor's group control, or lack thereof, since there appeared to be some issues keeping everybody in sight
- Your not asking the instructor to slow down, or feeling like you couldn't do so, even though that could have impacted the safety of the group
- Seeing if there might be suggestions for you on what happened, or things that only others could see regarding your diving
You seem to have eliminated many factors. Maybe you also need to have a head-to-head race with the teens just to get more data points?