I would like the ball to accelerate in the begging and end of the spline evaluation and deaccelerate in the middle, while it is in the air. Is there a way to change the default location?
You can evaluate the time at some interpolation whose derivative represents the velocity of the animation. In your case, such interpolation would look like:
Also, don't keyframe your nodes, try to use the Time Info and Animate Float nodes instead. Finally, you don't need the Mesh Info node, the same output is in the Mesh Object Info node.