Supposedly a myth.

Maybe. However, before I became a Mooney owner my bird was an Arrow
with a 3 blade prop. In that configuration, it absolutely did slow it
down quite a bit. I know that Top Gun Aviation (a Mooney Service
Center) just purchased one of the "new designed" 3 blades for their
shop Mooney. They want to determine what the cost in speed may be. The
manufactor claims that the new designs aren't slower, but we'll see.

-Robert