Well if you look in the code for a mob, there is probably some code that determines the speed of said mob. In your bow, you could modify that code. This probably wont work, but it could be something like (in your hitEntity method or whatever it is when the arrow hits the mob)
EntityCreeperAI.speed = 2F
That is theoretical code, you'd have to change it but I'm guessing that would work. You would have to make your bow fire a custom arrow entity if it isn't already though.