Getting passengers off quickly won’t help speed up the general post-flight process much. It’s very rare that the bottleneck is waiting for the cabin to be cleaned.
On a commuter flight with a 10-20 minute turn around time, deplaning speed is definitely relevant. Not many bags are being checked anyways, and the next set of passengers is already in line waiting to board.
On any flight > 1.5 hours or on a plane bigger than a 737, your point holds. On a 11 hour international flight via a 777, deplane speed is really unimportant at all.
Also, the US airlines will not hold a flight due to a delayed inbound unless it’s got a shitload of the passengers. I’ve seen Delta depart on time and cause 10 passengers on a delayed flight to miss by only 15 minutes.
I'd say deplaning quickly is about as important as boarding.