How fast does a paddleboard go?

A comfortable average speed is about 3-4 knots, on a regular all-round paddleboard. More wave-orientated boards are slower, while specialized race boards can be up to 8 knots fast. The ideal conditions (aside from surfing) is no wind days or get the wind behind you and do a downwinder. Riding against wind is a very slow process.
Downwinders mean getting dropped off somewhere upwind, and then simply paddle down to meet your friends downwind at the other end.