Wow. As the safety video often runs during taxi, or whilst waiting for push back I've never even considered it to be a cause of the delay?
I've always thought of that start of the video as an intentional "focusing" moment where it's quite probable that the video and cabin crew don't have 80% of people's attention, but by the same token, a few moments after a picture appears on a monitor and a few heads start looking that way, 80% of people will do the same.
I suppose another way to do it would be to have the cabin crew run up and down the aisles yelling "YOUR ATTENTION PLEASE!" but that would no doubt annoy me more... and actually would be likely to delay departure, as it would prevent them doing last minute things before they take their positions in the aisles.