I have noticed the phenomenon you're describing. Not a daily coffee drinker but, I do notice when I drink coffee I'm more relaxed and at ease which slows down my rate of speech (I tend to stretch my sylables) more on coffee and aides my fluency. Now if I drink too much coffee it makes no difference. There are times when I drink the "right amount" and still nothing. At this point I'm chalking it up to multiple factors that coincides with the day I drink coffee but yes I've had/ noted down similar experiences.