Poor lubrication would affect movement in BOTH directions, not making it any harder to hold the throttle open during cruising. Does it still snap closed when you release it?
I get the cramped hands too. I have a Throttle Boss (similar to the Throttle Rocker pictured here). They cost around $12.
It allows you to loosen your grip and press on the throttle with your palm.
It helps, but my hands still get cramped after a long day.
I've been told that wider diameter grips will help with cramping, but I haven't gotten around making that change yet (I intend to).
I hadn't considerd the strength of the spring. I would think you could safely swap in a weaker one as long as it's strong enough to snap the throttle closed when you let go.