Many years ago (early nineties, maybe), Boeing published an article on this very topic. As I recall, they ran the numbers on different techniques to “come down and slow down.” The results were interesting.
What sticks in my mind is that slats/flaps, unless you throw out a lot of them, provide little drag. At slow speeds, speed brakes provide little drag, as well, but more drag than the first or second increment of flaps/slats.
If my memory serves me (which is questionable), the “experiment” involved the 737....an airplane which has a reputation of not wanting to slow down and come down simultaneously. One scenario was throwing out slats/flaps at Vfe. Another was speed brake, alone. One was gear alone. Finally, speed brakes and gear. The conclusion was that the first increment or so of slats/flats, even at Vfe, provides less drag than other options.
In my career, I have flown for three airlines. The first was bought by the second. Retired from there to fly for the third. In all three cases, the SOP was to extend slats/flaps on a specified schedule. My first two airlines were union; the third not. It was the third where some guy with thick glasses looked at QARs for any and all “deviations” from SOP, including deviations from the slat/flap schedule.
Fly safe,
PantLoad