A 145 pound climber atop a 15 pound bike would loose 1.2% of his speed up a steep hill which is 45 seconds an hour and a 200 pound Clydestale on a 20 pound bike would be down by .9%.

On flat ground both riders would take an extra .1 kilojoules to propel themselves from zero to thirty MPH.

At 300W average the 145 pound rider would be at most 0.4% slower in the standing 500 meters dropping 0.2 seconds from 52.2 to 52.4 seconds (I'm assuming the weight is entirely where the tire meets the road because that makes the math easy). The Clydestale would be at most .3% slower dropping from 55.6 to 55.8 seconds.

The effect on flat ground speed would not be measurable assuming aerodynamics were the same; although higher weight on aluminum rims (DT 585 vs. 465/415 as in grams weight) often implies more depth (30.3mm on the 585 and 20.8mm on the box-section 465/415) which would make for a faster ride in spite of extra pounds.

Except for psychological impact (perhaps by way of things like sound) the loss would be effectively non-existent except in some racing situations, like a climber who was otherwise competitive riding off the front to an up-hill finish in a mountain hill climb. In general the body shape which makes a good climber allows for about 2 pounds of body weight per inch of height and I doubt there are many 8'4" guys or 6'3" women here. Mutant genetics can do the trick too although it's unlikely Miguel Indurain is among us.