My buddy bought a boat a few months ago with a CB foil already on it, we tested various props and then started changing the jackplate. His boat is better with a deeper prop to pad than most here and the foil was hitting the water at WOT. He took it off and gained 1.5mph, so yes sometimes they do indeed slow down certain models of boats. Many of the Champion boat guys that have tried them have had this same issue. On his boat, it didn’t add anything to the holeshot.
I didn't like the thing because I cut my hand on the edge while changing props, the edges are way too sharp.