From my experience speaking on a bad stuttering day does require a lot more mental capacity, leads to fatigue and saying nonsensical stuff, just to avoid tricky words/moments. I can imagine any other factor, be it mental fatigue or decline will make it worse. I do feel bad for him as I (as I am sure most people here will) have received comments of a similar nature. I'm not American but I would assume that many of his critics probably don't know about the stutter? That being said, he and Trump are both way too old to be president.