I remember Biden and Sanders in 2016; Biden actually appeared to be speaking more eloquently than Sanders. Sanders did not change like at all, and Biden - he appears to have declined very significantly, as if if he had stroke or something. Perhaps dementia.
I'd bet it has more to do with how little sleep candidates running for president tend to get. I'll be interested to see if he perks up after a few nights of really good rest. He can take a short break while his transition team does most of the legwork and brings him decisions to be made. He has no need to do any rallies for a bit.
His interview at his home was bad. He was way slower than he was in 2016. Something began to happen to Biden somewhere in 2018. He looks like 2 different people now and then. I do not understand animosity towards what I am saying: he clearly appears having decline, and if some hard-core democrats are willing to give and believe in excuses for that, many regular people are not.