The rules are pretty clear that if you lift more than your heavy load but less than twice your heavy load, you're limited to 5 ft speed.
https://www.d20srd.org/srd/carryingC...ingandDragging
Lifting and Dragging
A character can lift as much as his or her maximum load over his or her head.
A character can lift as much as double his or her maximum load off the ground, but he or she can only stagger around with it. While overloaded in this way, the character loses any Dexterity bonus to AC and can move only 5 feet per round (as a full-round action).