Actually, the majority of a ball's speed comes from the height of the initial placement of the ball in the stance (which represents the ball's amount of potential energy) which of course, most of which gets converted to kinetic energy on the downswing. This, combined with the
direction of the ball's trajectory at release , are by far the biggest factors affecting ball speed that is generated by the bowler. The forward motion of the legs do contribute to the overall velocity but alone, they represent only a small portion of the ball's forward speed. This subject has long been a topic of discussion. In fact, there was a great article
in the Nov 12, 2009 BTM debating this very question. Several well respected instructors
as well as a few bowlers had varied opinions. I had to concur with Bill Hall, who was one of the interviewees for this article when he stated that going to a lighter ball does not increase ball speed. I have yet to see a lighter ball, when propelled solely by a gravity-fed swing, result in an increase in ball speed. It confirms the
actual "physics" that is in play.
The important factor in this entire discussion is that we are talking strictly about a ball that is being thrown solely via the force of gravity. Any force that a bowler
applies negates all of the above.