Where is all this coming from about the family being unfamiliar with the PreCheck screening process? There was no mention of them taking out laptops, taking off shoes/light jackets/belts, etc. in the OP. The only thing the OP mentioned is that they pulled out liquids/medications which - depending on how much they actually had - could be entirely appropriate, even in the PreCheck line,.

Also, I am guessing that the entire thing did not really take 15 minutes. Maybe from the time OP entered the line and passed through security, but I seriously doubt the exercise of removing items, placing in bins / on conveyor belt, and passing through WTMD took anywhere close to that long.

As for etiquette, perhaps they could have told people behind them to go ahead while they prepared their items, or the people behind them could've asked if they could just jump ahead (which I've done). Regardless, neither of those things happened.