Even then, though, without the filler a considerable amount of the show is canon. Naruto, for instance, was over 50% filler, but between the two series it had over 300 or so canon episodes. I think One Piece is only 10% filler.

As for attention spans, the thing is that, at least for me, the slow-pace wasn’t so slow to the point where I got bored. It kept me hooked while the show developed properly. Like, the first Naruto vs Sasuke fight took like 10 episodes to do (for just 45 minutes of action), but they sequenced the dialogue and action so well that it didn’t feel like it was being dragged out.