Mate you ever rowed a boat? That shit is hard work, it goes below the bike for sure
Of course, the pedalo manages to combine the worst of both worlds and seems to exist solely as a way to work your quads out without looking like you're doing a gym session
If you have good balance, I think paddle boards are more efficient than a kayak, assuming your not carrying a load. You can put your whole body into the stroke. Kayaking destroys my shoulders
I'm definitely biased here since I spent years kayaking and have only gone paddleboarding twice, and I also definitely don't want to just be all "lol skill issue", but I think this might be an issue with your (kayak) paddling technique. You should be using your whole torso rather than your arms and shoulders
Anecdotal information from my observations only: I have two paddle boards, and a tandem kayak. When we go out, the person(s) in the kayak are always able to go faster with less effort - even if they are solo. Rowboats with oar locks are generally faster still, and none of us can even hope to keep up with one of those Hobie kayaks with the pedal driven mid drive.
I agree in perfect conditions, but paddleboards are completely impractical as transport. I once got blown out to sea by unexpected high winds while on a paddleboard and I was lucky there was a boat going past to pick me up. Kayaks on the other hand can handle rough seas and carry many days of food.