My 5" shield cut fletching also makes my arrow act stiffer than the corresponding, slightly lower, 5" parabolic fletch. I might be right on the border line of "too stiff" and that little bit of difference in fletching area pushes it over the limit. I only know I can shoot about 50gr more up front on the shield cuts and get the same arrow flight. The lower parabolics SEEM (I don't have a chrono) slightly faster, but I shoot them both and since I'm nearly stone deaf, I don't worry about it.
My hunting arrows have 5" shield cut with a lot of broadhead weight up front and the parabolics are for my 3-D shooting with about 50 gr less point weight.