I think it can be action/stiffness vs weight that tires you out. Very stiff rods tire my arm by the end of the day because of the extra hard punch/stop needed for a good loop (which admittedly is very tight). But I've come to prefer what's most commonly referred to as medium fast so the rod does more of the work and they seem to fish better. I've fished 6 wts that were easy casting and 4 wts that tired me out.