I can get close to 50 mph (safely) downhill on my 30 year old steel framed 8 speed bike with 20mm tubular tires pumped to 120 psi. No problem with riding at 20 mph on the flats and I'm close to 60 years of age. My "aero" CF bikes with deep carbon rims are minimally faster. Speed is irrelevant unless you are racing. Enjoy the ride and stop relying on equipment for minimal performance gains.

Its totally possible to hit over 40 mph on steep hills on an MTB, especially if you weigh more. I was more referring to slight grades with just pure rolling without pedaling.

Im in Austin so I frequent COTA Bike nights on Tuesdays on the track. Coming down the hill from turn 1 with other riders is a very good test of aero efficiency, because there is a slight uphill after the bend. Couple of my friends ride gravel bikes, one of them is heavier than me, and its pretty clear that that my bike is more efficient.