Thinking of those I have encountered who I would say have gravitas, I’d say less is more.
In general discussion, don’t stray out of your area of competence. If you don’t know, say so; if you’re guessing, say so. DO NOT BULLSHIT.
Not sure I agree with the speaking slowly bit. I’d say speak naturally, but don’t speak much. Make sure what you say is well thought through.
Doing a brief or presentation is a little different. Be well prepared, expect interruptions, and think about how you’ll adapt when they say you’ve got 5 minutes instead of 20.
The confidence others mention will come from knowing what you are on about and only saying things which are well considered.