I read that just slowing down enough to increase flight time by a couple of minutes could save millions in fuel costs.
A flight cruising at roughly 0.80 Mach (roughly 525 mph with no wind) at 36,000 ft, going 3000 miles, slowing down 1% will lengthen the flight 3 minutes and reduce fuel consumption by, I would guess, 2%.
The passengers will not notice three more minutes on a nearly-six-hour flight, but the airline will definitely notice a 2% fuel savings.