Originally Posted by
noglider
I do see some people on the bike path going faster than I can imagine going. They have no cargo, or they have a small backpack and no rack/luggage on the bike. I imagine some are commuting but most are just riding. When you commute to work, you generally have stuff to carry.
On the flats, once you get up to speed the extra weight will not matter; the extra bulk might make a little aero difference. So somebody flying past you is probably just because they're putting out more watts, not because they're carrying less weight.
Downhill the extra weight will be an advantage, uphill a disadvantage; downhill any extra aero penalty will be more severe, uphill it will be less relevant.
It would be interesting to study the conditions that would make cargo be a net advantage downhill (extra weight pushes through air resistance, more than the extra bulk causes air resistance). The ridiculous extreme of a sail or parachute would be almost no extra weight with lots of extra air resistance for net slower downhill, but a rack and rack trunk out of the slipstream would probably have negligible aero difference, and net benefit for downhill from the weight.