'Speed' boards really only really come into their own in stronger winds and speeds over high 30's. (Unless you are smaller or larger than 'average'

). So unless you regularly get winds in the high 20's and over, you will probably go just as fast and have more fun on a smallish slalom board (with approximately 10-15 litres more volume than your body weight). Most of the top 80-90 litre slalom boards are quite capable of 40 knots in the right conditions, although when they are, you could likely gain a knot or three in a smaller 'Speed' board.
There are a few 'Speed' boards in the 80-90 litres size range and in broad reaching conditions on flatish water, they are probably slightly faster than the pure Slalom boards, but the extra versatility of the Slalom boards would make them a lot more fun in most conditions. More riding = more fun!