Offizieller Kommentar
The speed slider is essentially the mission speed limit. If you have it turned up to 15 m/s it will go 15 m/s on the approach and return.
But, since it is a Custom Camera setting you may have to raise the "Max Image Rate" to something like 2 or 3 seconds per image to get the aircraft to slow down while on the planned path. The default value is 1 image per second which usually allows for faster flight than the 15 m/s speed limit so the speed on the planned path gets pinned to 15 m/s too.

