Rocker, board weight, and length rule. You probably don't have to worry about rocker, since any shaper doing race boards, already has it dialed in. That only leaves length and weight for the buyer to decide.
It is worth paying extra for light weight when it comes to gliding.
It might seem the opposite, more weight equals more inertia, but it doesn't seem to work that way for getting on a plane. You stand a way better chance of building up higher speed and breaking into a plane when a board is light.
Width I don't see as that big an issue. If you can break onto a plane, you beat anyone not planing, regardless of width.
I think you can reach a point of diminished returns going too long and adding more weight. Depending on the conditions you're trying to go fast in.
Glide is all about displacement hulls and the ultimate factor is ... waterline length, and a nice long slippery hull that sits down in the water.
Look at racing sculls, yachts ... and ships.
I can't imagine a paddle powered craft getting up on the plane on paddle power alone. The less than pleasant thing about SUPping on flat water is when you are paddling fast and you can feel the bow wave holding you back.
Try a simple experiment, put a very small fin in, then see how fast you go in the direction intended. If you have a GPS with heart rate monitor use this to paddle at a constant rate, then put a huge fin in & note the velocity made good when you paddle @ the same rate.
Play with your fins... because of the one sided paddling , an appropriate sized fin that will keep you tracking straighter for a longer period will give you a little more glide FORWARD.....minimising the yaw .
..they're just the main ones. imo.