All of the above contribute to the decrease in the speaker's credibility when delivering a speech.
A speaker must provide the audience with evidence or reasoning.
A speaker must not exaggerate; that leans towards desperation to be believed, which can be taken as loss of credibility.
A speaker must not be emotional during a speech if the speech is meant to be formal/informative/persuasive (anything other than personal).
I hope I was able to be of help.