I think Biden's stutter is a big factor here that his campaign has done a shit job of staying in front of. Halting speech, word finding issues and losing train of thought are common for someone with a stutter. More specifically, they are common for Joe Biden. He looks worse than before, but under stress and fatigue, stuttering symptoms get markedly worse, and the stakes are currently fascism vs. democracy.
CNN not fact checking left Biden with very little time to rebut a firehose of lies, and Biden's advisors tried to have him rebut each one. They should have focused Biden on picking the Trump lie he can work with the most, and rebutting that one lie thoroughly each time. Biden's advisors stupidly ran him ragged in debate prep, cramming useless facts for a debate that was always destined to turn into an insult-fest.
All that created a worst possible case for someone with a stutter and it will have ripple effects for a long time. Every post-debate appearance just got much more high pressure and stress.