Right. The effects are less than linear since aerodynamic drag scales with the square of velocity.

Consider a 200 pound rider with the frontal area of a 5'11" professional cyclist riding on the brake hoods up a 3% grade. Doing a little rounding converting to metric numbers we get to 90 kg, 12 MPH, 5.3 meters per second, .400 meters^2 area, and Cd .760 per Gibertini and Grassi.

On OK clinchers and an asphalt road he'll take about 190W to maintain that pace.

An extra .9Kg (2 pounds) will drop his speed to 5.27 meters/second which is just .6%.

Of course on steeper, slower climbs the results will be closer to linear and just 1% (36 seconds in an hour) can still be more than enough to win or loose.