This is for the emergency descent case as in depressurisation. In the old days on 707/VC10 type equipment, it was a discussion point as to whether it was better to commence an immediate descent with speedbrakes out or slow first to gear lowering speed with speedbrakes out, then lower the gear and let rip. The problem with the 747 is the transition with gear extending/retracting when extra landing gear doors are open and it's not a good idea to blast them with 550 mph airflow. Nowadays, we don't bother with the gear- with full speedbrake and accelerating to near IAS speedlimit, you get down as rapidly as you can handle with all the other things going on at the same time.