[Schroedingersat]

When biking having leftover reserves for the hill or the traffic lights saves a lot more time than using it up on the flat.

The actual output between a moderate effort and going as hard as you can is a 20-30% in terms of wattage, but that only translates to 5%-10% speed increase because the power required to overcome wind resistance scales with the third power of velocity. If that comes at a cost of a 5% power decrease (where power is proportional to velocity and you spend a greater amount of time per unit distance) on the hill because you're exhausted then the gains can wind up being minimal.