Since his run for 2016 his speech has been teetering on coherency... It's part of how he became a meme. How far gone he/it is is more or less meta and par for course.
He drifts between topics more regularly and in fewer words that he used to. Like in 2016 he would say dumb stuff, but could say two sentences in a row or more on the same subject before switching to something else tangentially related if you squint hard enough. It was word salad, but you could follow it if you just pictured it was the utterance of an excited four year old.
Now he has trouble finishing sentences without switching subjects and instead of saying nonsensical things it is more of a word puree than word salad. Everything is mixed together and completely incoherent instead of stupid. It was a low starting point, but it has gotten worse.