Ed Ashby found that a helical fletched arrow will make one full 360 degree rotation every 60 inches in flight. But a single bevel broadhead will make the same 360 degree rotation in around 16 inches of tissue penetration.

If your fletching matches your single bevel (left wing fletch/LH bevel) then the rotation of the arrow and broadhead will actually increase speed on contact with the animal. The arrow speed in fps won't pick up but the speed of rotation will.