Witcher 2 Encumbrance Bug
So I'm beginning to question whether this is truly a bug at all.
Regardless, I found out how to resolve the problem; Simply continue along the forest path until you meet Iorveth and Triss puts up the arrow shield. At that point, assuming you're currently unencumbered, you will regain your normal speed.
This is normal. Until you reach the cutscene with the elf you're forced to walk only even if you are not encumbered. I had 500 pounds of gear and waited until after the hanging to sell it all off to a merchant (fairly easy to fight under the shield while encumbered).