Working with Paths programmatically can be complicated. To help you get oriented with them I would suggest going through the vpPath samples found at
\resources\samples\vegaprime\vppath\
Take a look at the sample called vppath_lookat because in it, it appears they are changing the “initial speed” between waypoints. There is a method of the class vpPathKinematicState called setUserInitialSpeed(). It appears this methods sets the speed at which you start a segment in a path. This is done inside the configure() method. I have not tried to do what you are asking.
Another thing to consider is the AI.Implant module. It is an addition fee but may be well worth it if you want to add some “smarts” into path following. It will do precisely what you are asking. Other wise you’ll have to figure out how to do it in code.
EDIT: Another thing to consider is instead of trying to modify the speed, instead think about modifying accelerations and decelerations triggered at the points you desire. I don’t use the Path tool because A) its buggy and B) it doesn’t work for LADBM terrains.
Side note: I found out from Presagis that the official stance is they have no plans to fix the Path Tool and its many bugs such as not working with LADBM. Isn’t that nice. 
Steve
