I can only speak slowly if I have notes to work from. In normal conversation, including work discussions among 2-10 participants, I tend to rush to get my thoughts out, and that makes it hard for people to follow.

I've found that having a whiteboard helps a lot (a notepad on which I can scribble less so, but still better than nothing). Having to slow down enough to write main points down or sketch out some things seems to make me more comprehensible without losing my train of thought.