While descending, when is it faster to keep pedaling, and when is it faster to just tuck?

Here is the particular descent I'm thinking of, well, this is the climb, but it's the same thing, just in reverse.

When I was riding with really fast friends, I remember we'd be going down some moderate inclines, like 2,3,4%, and they'd be in the drops, pedaling like a madman...

On this descent, because it's a bit steeper in most places, I'm often at a place where in my hardest gear (I believe it's a 52/10), I can still feel a little bit of resistance, but not much.

The last time I did this descent, I basically pedaled all the way down, as long as I felt any resistance at all. This time, today, I focused more on tucking and getting aero.

Tucking more and pedaling less, it was about a minute slower, 12:01, vs 11:07. But also, wind conditions were probably different, and I'm sure that has an effect too...

So I'm wondering, in general, on a descent of 5-6%, that's not super technical, just smooth curves, when is it a good idea to keep pedaling, and when is tucking faster?