Thegain in speedwith straight fletch, is tiny. Thinking that the arrow will be more accurate with smaller fletch is also false. Fletch is there to stabilize the arrow. So if you inflict some force at the moment of release, fletching is going to bail your arrow out(lots of other variables too)

The more tuned a bow is the less fletch is needed to stabilize it. The better aerodynamic the head the easier it is to control with less fletch.

If you grew up back in the day when we still shot big huge fixed blade heads, and finger release with compounds, you'd certainly understand the desire for helical. Helical was your friend. Still is in the trad world.