Paul,Do we have any measurements for the speed on the end of the line (fly) when fly casting?
If you think the fly and later sections of the line tend to follow the path of the line ahead of it (and I think this photo of the fly path in Aitor's late tail video tends to show that is a reasonable (if not exact) assumption).
Then unless slack forms in the fly leg I would expect the speed of the fly (and any other section of line in the fly leg) will be about equal to the speed of the line at the rod tip at a given point of time as the tip goes to RSP1.
As has been discussed in previous threads others would disagree with this first order assumption.
Gordy