This is due to Bioware implementing a unique movement speed for player characters that is faster than most creatures in the game. The PRC has a function to even the playing field between NPC's and PC's in this respect. If you would rather go back to how Bioware implemented it, toggle the following switches to 0:
PRC_PNP_RACIAL_SPEED    - Controls the speed for large and smaller races.
PRC_PNP_ARMOR_SPEED    - Controls the speed for wearing heavier armor.
PRC_REMOVE_PLAYER_SPEED    - Controls unique player speed, changing it to normal speed.

Could you or someone else please explain where to find these "switches" and how to toggle them off?