What demands? It's his choice to do all those rallies. And he's energized by them. He doesn't show up downstairs in the Oval till eleven, and apparently he's back in the Residence by five or six at the latest, unless he's entertaining.
So I don't think it's the demands of the job. As many people how been pointing out for several years, the Trumpty of the 80s and 90s had a much larger vocabulary, and was far more mentally agile.