Yes, but the physics is that the amount time saved over a fixed distance is actually more for the 15mph cyclist than the 20mph cyclist. However the percent of

Yes, but the physics is that the amount time saved over a fixed distance is actually more for the 15mph cyclist than the 20mph cyclist. However the percent of time saved for the slower cyclist is less, but only a little. Basically at cycling speeds the aerodynamic curve is pretty flat so it doesn't really matter if you are fast or slow.