Fastest way to get down in the 744 was to lower the gear and use the speed brake during the descent. Issues to consider were the rising min manoeuvre margin (yellow line) on the speed tape.
The answer for me was to lower the speed brake (stow it) as you get near the gear out speed, lower the gear and simultaneously start the descent in FLCH to build up speed. You needed to get momentum as once that airspeed started to bleed off, it was hard to get it back.
Once the descent was underway and the speed building up, pop the brakes out to 'flight' and sink like a stone.
Any turning would see rising of the min manoeuver margin on the tape, but that was ok as actual airspeed would build up during the descent beyond the min manoeuvre and gear extension speed (extend speed was above extension speed).
The above action was for a rapid depressurization, not a fire in which you might not give a toss about the gear extension speed in the first place. It was also found unfavorable for the depress case and, soon after I joined that company, the procedure was removed from the QRH as an option for Rapid D descents.
Given we're considering depressurized flight at 25,000' after a fire warning in a cargo aircraft while motoring at best speed towards a landing zone (preferably a suitable airport, but not essential if still ablaze), then I guess there is a lot to be said for:
1. follow checklist
2. depressurized flight at high speed to a landing 'zone'
3. if actually on fire, flames confirmed, time since warning +10 mins
4. big call, but must get on ground or water asap
5. if not confirmed on fire, fly to landing zone at FL250 fast as you can go
6. on reaching TOD for selected landing, slow down
7. lower gear
8. simultaneously speed up and descend
9. when suitable margin exists beyond min manoeuvre margin, extend speedbrakes, dive toward airport, extend flaps on schedule or land flapless.
Tough call.
Whatever happens, there'd be no need to keep it pretty.