He was never a great speaker even in his younger days, often mixing up names and making embarasaing gaffes (like introducing Obama as “Barack America” or asking a guy in a wheelchair to stand up), part of it is also he is a stutterer from childhood and there is a particular way he deals with getting a word stuck in his moutn. I can understand how listening to him can be jarring for foreigners, particularly if you’re relying on short clips and are not used to the way he speaks.
But there is a world of difference between that and actual cognitive decline - Alzheimer’s or Parkinsons or whatever else people imply that he has - all of his medical records are public, he is being constantly monitored and those things are simply not there. He does have a neurological issue with his legs (from an accident many years ago, I think), which is why he walks a certain way. Again, all of this doesn’t matter - FDR won the WWII from a wheelchair, the president job is understanding the situation fully and making key decisions and Biden has been an incredibly successful president in terms of passing legistlation despite the divisions in Congress. Thre is nothing his critics can point to where his supposed deficiencies had a negative impact of his ability to perform his job.
So the whole narrative about him not being up to it is a distraction and replacing a candidate at this point would pretty much hand the election to Trump for a myriad reasons. The people you hear complaining are mostly rich white guys like Clooney who would be OK regardless of the outcome; the base of the Democratic
Party - African-American voters, organized labour etc - are solidly with Biden and more so than they would be with any alternative candidate. They will say Joe stumbles when he speaks, but his heart is in the right place and he gets things done. Again, it’s hard to appreciate the dynamic at play from the outside and I can understand how after someone like Obama you might think how come United States cannot get someone else just as charismatic to represent them on the world stage - but the truth is Biden has been a much more impactful president than Obama even if he is hard to listen to sometimes.