Question posed in top slot:
"...Why do you save fuel going slowly ... in descent ? The aerodynamics behind it ...?"
It`s not a question of aerodynamics! It`s a question of fuel flow, as I explained in the 2nd post! It`s a question of gaining more or less time with engines on idle.
Try this...
Do one flight one day, using descent speed 250 from TOD till approach. Take note of the number of minutes and fuel used from TOD to landing.
The next time make a descent at 330 Kts from TOD till approach. Take note of the same figures!
Then compare... you will see why...