Well, you would think wrong.

I shoot a strong helical with feathers, through mine and when crono'd, it looses 4-5 fps. With straight fletch, it is 1-2 fps on my set-up. It does vary with how the biscuit is set-up. For the best results, the hole for the arrow must be large enough and the biscuit must be perpendicular to the arrow.

I've tested biscuits that I've modified many ways. I've trimmed the back, cut notches for the fletch to pass through and widened the arrow hole to various degrees. Interestingly, when I cut notches and trimmed the back the arrow speed averaged 1 fps faster than when using a two prong rest.

Let me say that without doubt, when properly set up, speed loss in not an issue with the WB. I'm sure there are a few speed merchants who do anythng to eek out an extra foot per second, regardless of the effect on shooting accuracy, but for all practical purposes a couple fps has no effect on anything important in archery or hunting.