I used to talk fast without intending to. And I realized it was from a fear of blocking on words. In the end though, I still ended up blocking on words. And what's more I was out of breath because I was speaking too fast. I didn't feel in control or calm when speaking so fast either. Slow controlled speech with word elongations is better imo. And add plenty of pausing between 3-6 words or until you feel you're about to run out of breath. And breath using your stomach, inhaling slowly through the nose and talk when releasing the air out through your mouth. It takes practice but its more calming when speaking this way.