Borderlands 2 running glitch on one of my characters (Zero)

potatoes are the problem.

try verifying your files maybe.

Some weapons or perks can affect movement speed. I hate to say something that might be totally obvious - but just in case, make sure that you don't have the bane SMG equipped...

did you equip one of those huge shields (i think they are called turtle)? they lower your speed.

edit: it's a legendary called fabled tortoise