As the title says, using the curve as a start speed doesn’t seem to increase/decrease speed over a lifetime but rather use some static value closer to curve’s x-axis 0.
Well it should be pretty obvious that using a curve for the start velocity of a particle doesn’t work / makes no sense. The start velocity is applied at the time a particle is created. At this point its current life time is zero. The start velocity is applied only once. You should look into the other velocity modifications like “Velocity over Lifetime”, “Limit Velocity over Lifetime” or “Inherit Velocity”: