I think you'll find sailboards can't keep up on a proper course, they
are damn fast on a reach and not in any other direction.

Andy.

Andy,
there's this race in San Francisco, USA. It's between two bridges,
dead downwind. Anything powered by wind can enter. Kite boarders,
18ft skiffs, windsurfers. 2002 Kite boarder won, followed by a
windsurfer 2nd, followed by an 18ft skiff. 2003 Windsurfer won,
followed by an 18ft skiff.

Ronstan Bridge to Bridge, SF, CA

The skiffs do ok upwind but are still behind the windsurfer at the
upwind mark.
I proved that to myself today. There was some racing today on
Biscyane Bay.
420s, 29ers. I let the 29ers clear there start line. I sailed after
them on my windsurfer. Upwind downwind course proper. When I rounded
the upwind mark ahead of them, I waited till they rounded and passed
me, then I started for the downwind, past them all and was first to
the downwind.

AC boats would win the upwind in less then 10 knots. 10knots the
windsurfer would be waiting at the mark for the AC boat.

fastest production monohull (non-displacement , non -sailboard)?
My money would be on the 18ft skiff.