I think that most of us with SA talk fast because we don't like being the center of attention. We want to get it over with as soon as possible or maybe we feel like we're wasting the other person's time if we talk to slow and they won't like us because of it.
Either way, these are ANT's. Being the center of attention is really no big deal, your negative thoughts just make you think that it is. It's unlikely that the other person will think negatively of you if you do speak slowly and calmly. If they do, then that is their problem and not yours, so maybe find another person to talk to.
Maybe it will take a little time, but the other person might just start speaking more slowly because they are picking up on your vibe.
I agree with @anxiouspa, just because someone else is talking fast doesn't mean you have to. I think that slow talk is one of the methods that has helped me the most and it has gotten better and better with practice.