I see my GPS taking me cutting through islands and marshes so no clue how accurate the speeds are. Does track my car for miles at 60 mph on cruise.
“but really couldn’t cope with the tiny cockpit”
Fit makes no small difference in forward stroke mechanics.
If you would otherwise have better form and rotation, too snug of a fit will, not might, interfere with efficiency.
The Solstice GTS is the low volume version. There isn’t any room for thick thighs in that kayak, and although it’s got good overall length, the waterline length is somewhat shorter. It doesn’t carry significant volume to the ends. It has fine ends, with a convex curvature leading from the keel line to the deck at the ends. (In the older version - the newer version did away with this.) So I don’t think it’s a terrifically large-volume kayak. It’s fish-form, the older version had a peaked deck, and that’s as far as most folks ever get in their analysis, other than to worry if that peaked deck is going to cause weathercocking, which is the opposite of the truth. But it is something I here over and over and over anytime someone finds a couple extra inches of deck height than they personally don’t think they require.
In any case, I’ve done some similar trials, just using a stopwatch with a marked starting and finish point. I found much more consistency in my times between I believe 6 different sea kayaks. The biggest difference maker was in the fast runs. Something I discovered is that I really get tuned into a cadence. I think this is where all the heart-monitor measurement comes in - although that will have its flaws too. But if I keep the same cadence, and just work a bit harder in one boat than the other without paying that much attention, it gives the appearance of the same speed.
But your results are quite different, to an extent that I would think forward stroke form is involved.