Sure, it's taken the wrong way when one is already speaking/stuttering. It's such a personal thing to people and when anybody tells you to "slow down" then it can seem offensive. But, in hindsight, it's the best thing to do. It's almost as if the other person is giving you permission to speak slowly. However, after saying all of this, it's best to avoid telling somebody to slow down (while they're stuttering) because of the emotional consequences.