RE: how much speed is really lost from a biscuit
Back when i used to shot one i was seeing a 2-3fps drop with a helical fletch and a 1-2 fps drop with straight fletch.
The speed reduction will be increased if the disc is too tight around the shaft or the biscuit isn't tuned properly- it shouldn't be correcting arrow flight, merely giving the arrow something to sit in and slide on when it's on its way outbound.