I’m trying to have 2 curves:
 1 for the spline that the camera will follow
 1 for the speed of the camera
How it currently works:
 At each frame n, I evaluate the 1st curve for n, and the camera’s position is set to (n,f(n)). So currently the velocity is not implemented.
 I have created a python script that creates the spline mesh from the fcurve.
My problem:
 Implementing the velocity curve. I think I need to do some math with the curves but I’m not sure what or how. With default velocity, we set the position to (n,f_1(n)). But with a different velocity, we need to find out what input to give to $f_1$
How I think it could work:

The velocity curve has one controlpoint for every controlpoint in the 1st curve.

The curve between any two controlpoints n1 and n, indicates how fast the camera will travel from the first controlpoint to the 2nd one (in the 1st curve, which is the path)

For example, at frame 130, we don’t want (130,f_1(130)), but we instead want (130,f_1(160))