It looks like some of Trump's speeches that are incoherent just involve him not knowing WTF he is talking about and half-heartedly reading what's on the prompter--e.g., when trying to speak to some technical points on the COVID-19 situation--or doing some of the NLP stuff that looks weird but seems to work (e.g., repeating phrases), whereas some of Biden's issues, where he's falling asleep on camera at a "virtual rally" or being incoherent while saying complete nonsense, look more indicative of dementia or other age-related health conditions.
Not trying to do the usual partisan crap here. I can understand why someone would look at Trump and point to a lot of the things he says seeming incoherent. Some of his recent press conferences have really sucked, with him reading talking points in a lame, monotone way, and saying some weird stuff about "beautiful tests" and garbage like that.
One day, though, I left on a network that actually aired one of his press conferences in full and was surprised to see him say things about a variety of topics (farm subsidies, some details of legislative proposals, ventilator production, etc.) clearly and lucidly once he was just speaking and not just reading some talking points someone prepared. It was the one on Good Friday when he was there speaking for a couple hours. I was pretty surprised. Of course, Biden is capable of speaking clearly and lucidly most of the time, too. I'm worried about Joe, though, in all seriousness. I have a hard time thinking he could get through even one term. Of course, Reagan pulled something similar off for quite a while.