Undercarriage doors.
The drag from gear doors used to be quite an issue on the early 747-100 with its underpowered & initially unreliable P&W JT9's. Selecting gear up cost you about 3kts as the gear doors came down to retract the gear. Of course a quick Flight Eng would want to start fuel dumping, but that cost another 3 kts in the vortex as the fuel flowed out of the pipe at the end of the wing. So 6kts in a heavy underpowered 747 on 3 eng was not too good! Engine failures were so common the boys got quite good at this. I am told early captains waited for V2+3, gear up. When gear lights out & V2+3 fuel dump. Remember this was a marginal situation & fuel dump was considered essential to survival then in the 1970s. Todays -400 bears no comparision, even on 3 it leaps ahead of V2. A skill & understanding may have been lost.
By way, the modified JT9D became very reliable, strong & robust. A superb engine!