first report new springs with a surprize result

As a pilot with a degree in aerospace engineering I can tell you that “parasitic losses” can manifest themselves in many weird and wonderful ways. Likewise the gains created by eliminating them, witness the little air tabs (originally designed for aircraft) that can measurably increase fuel mileage on a coach.

In this case, I’d say an overloaded suspension created excessive (even if you weren’t aware of it, the slow creep is imperceptible) motion which created the losses you’re now recouping.