It's well-meaning advice but comes from a complete lack of understanding of the problem. For me, later in life, I do actually find it helpful to slow down in order to gather my thoughts, consider word choice, think about how to physically get my speech muscles to do the correct thing to say what I want to say. So I'm not slowing down to magically make speaking easier; I'm slowing down for a specific purpose to create the right conditions that allow speaking to happen more easily than it would otherwise. But this is after years of dealing with the psychological and emotional stuff that used to get in the way. Slowing down made it worse when I had all those problems because my mind would focus on those rather than the act of communicating. My mum once actually told me to speak quickly. That didn't help either because my speech muscles couldn't keep up. Speak at your own pace, as it suits you best in any given moment.