With most manufacturer test results recorded at 40 or 50 kph (this new thing saves X seconds!) I've always wondered if riding in an aero position made much difference at my slower pace (low 30s these days). Turns out Specialized tested exactly that in their wind tunnel recently:
I was surprised to learn that the "hoods vs drops" test at 50 kph and 20 kph yielded an almost identical time saving of 15% over a 40k distance.
I guess the change in wind sound is just a lot more noticeable when you're in an aero position at higher speeds....
This video in the same Specialized "Win Tunnel" series does a great job of explaining why aero savings at slow speeds matter:
Basically, even though you're saving less at slow speeds, because you're spending longer riding the same distance vs a faster rider, those small aero gains add up to around the same as the bigger gains over a smaller time.
I know I'll be spending more time in the drops even when I'm soft pedaling!