Maybe you can cheat and put a run speed enchant on that char. Don't forget some run speeds come from weird places (gems, trinkets, gear, etc.). You already checked most things people will mention. So might check if that char is using an addon that slow things down. My main shammy (not in a group) had a corrupted totem setting (for a totem addon) that lowered frame rates down to single digits. I deleted the settings file and things went back up to high frame rates. Since the addon doesn't load for other chars, they never saw it. Maybe there is a talent (unholy) that affects run speed (like the blood, frost and unholy auras used to be) that even when not in unholy stance the other chars move fast, but the slow one isn't speced into so it runs slow. Just guessing. Hope you find it.