I'm not convinced that Biden hasn't slowed down. Dude's old but his speaking style, history of gaffes, and his stutter makes it hard to really gage a change in his public speaking. I definitely don't think he's struggling nearly as much as Trump but ultimately I'm not sure how much difference it makes for how I will vote. Even if Biden was as bad as Trump he would be the better choice because he builds teams and listens to experts whereas Trump collects sycophants and listens to whoever best flatters his ego.
The job of the President is complex and involves dealing with incredible amounts of information. No one person can meaningfully process the amount of information Presidents get presented with everyday nor have the background to understand and properly contextualize the variety of types and sources of that information. The person that recognizes that they aren't experts on every subject and who builds teams of subject matter experts to help them process the information and make the most informed decisions possible will always be the better choice.