Totally NOT a waste of time… and if it was, so what… for most of us, that’s what we’re here for… to waste time!
A few things to look for:
I doubt it has much to do with the fin setup. Pulling the back fins off the rail and closer to the stringer would make it looser, if anything, not stiffer. But since that’s the only thing you can adjust at this point (aside from putting in a trailing box) you should try to address the issue there as best you can. I’d say (as Ken said), push the cluster together (front fins back, and back fins up), or… move all the fins up in their boxes all the way, and see if that helps. Bigger fins in front, and smaller fins in the back is another option. All worth a try…
But I think your problem may have more to do with your bottom contours and/or tail rocker. Your single flattens out at the front fins, then the double starts? If that is true, you may be creating a lot of drag where they meet, particularly if your outline, rail profile, and rocker don’t accommodate such a setup. Typically, the single would not flatten out until the trailing edge of the back fin, and the double is shaped into that. This allows the water to flow smoothly through the concaves, allowing them to do their magic. So check that, and see if your single really does flatten out before the doubles start. And 2" of tail rocker is OK… not a lot by HPSB standards, but OK. But where is the curve and what shape does it have? Is the “break” too far from the tail? Or does the tail rocker smoothly acclerate out the back, creating a little “kick” around the trailing fin.
I’d also suggest you take a look at your edges… are they crisp and long… soft and short? How does the top of your rail through the wide point and back blend into your rail profile in the tail? If they’re shaped very differently (round to square) the transition has to be very smooth, long and fluid, and match the foil of the rail. Shape and volume have to change together. I believe changing shape and not volume, or volume and not shape, can crate a lot of drag, too.
Finally, I’d suggest that your overall volume, if it’s much greater than what you normally ride, might make it feel stiff. Not slow, but stiff… meaning harder to get onto a rail. A wider outline can have the same effect.