I noticed the same thing with shrooms and I was able to get a consistent improvement by practicing my speech when on shrooms. I could feel my jaw muscles and all the muscles that go into speaking and breathing much clearer and could feel that my jaw was super tense. Doing stretches extending my jaw helped. This last trip was about a month ago during a period of extreme stress for me where my stutter was the worst it's been in years. I'm back to my "normal" level of fluency now where my stutter is not that noticeable.