The amount that it slows an arrow will vary depending on the setup and arrow configuration.

If you're shooting fat aluminums with a big helical twist on vanes, then you might see something in the 10-15 fps loss. If you have small diameter arrows with straight or slightly offset fletching, then you probably will fall in the 1-4 fps loss range.

I've experimented with many different setups on the WB and have found the following. If the biscuit is too tight, speed loss will be greater. The rest is designed to be shot with the bristles only touching the arrow on the bottom when the bow is drawn. Mine are always setup very loose - you might say sloppy.

I've actually tried to see how little speed loss I could get out of one, so I made the following modifications. I trimmed the back of the biscuit, so approximately 30% of the bristles were removed. I then cut slots for the vanes to pass through (this was on a target arrow with slightly offset vanes ). This involved removing all the bristles in three 1/2" sections of the rim. When shooting this setup through a crono, it was 1 fps FASTER than a traditional 2-prong rest. By the way, the WB shot great this way and I used it for a full year, but it did wear out faster.

Bottom line - the speed loss is insignificant for hunting. If you don't want to use it for hunting, pick a different excuse.