I know that I speak very quickly. People always used to comment on this in school, which just made it worse.
I remember being in drama class and being asked to narrate the story, and the actors couldn't keep up. I didn't realise until I got to the end of the narration and noticed some people laughing, others looking frustrated and the teacher going "You did that far too quickly. How about slowing it down next time?".
I made a conscious effort all through school to slow down on the rare occasions I spoke, but the comments remained the same and so I don't think it worked. I've since given up trying, and nobody's ever commented again but I imagine I still do it.
I think I also have a slight speech impediment, but I don't even know what it would be called, and I don't think that helps. To me, it feels like my top lip is too tightly pressed against my top front teeth when I talk, so it's slightly like a lisp but because of my lip not my tongue. And I do the too loud/too quiet thing.