Tightslot
It seems counter-intuitive but a similar effect is seen in corridors and door ways. Placing an obstacle upstream of the opening or some railings along the middle of the corridor speeds up the flow, something to do with reducing the tendency of people to jam themselves together.
As for the briefing, experience has shown me two advantages.
1, Airline equipment differs, e.g does the life jacket have one red toggle or two. tapes to tie or a Fastex buckle?
2, Cabin crew remember you taking notice of their briefing when you need to ask a favour later in the flight.
Question for frequent fliers: What has changed in BA's briefing recently?