Hopefully just a bad day. It was stated before the debate that Biden had a cold and had done a covid test. Shoulda probably postponed it but probably thought the optics would look worse than how he thought he'd perform. He was clearly hoarse, possibly feeling like shit.
There's at least as many health concerns about Trump, although I'm not sure he's declining, hasn't changed much over the last 6 years or so, except that recent foaming in the mouth thing. He no doubt takes stimulants, not only because of the accusations/projection sent Biden's way, but he probably has ADHD, thus a legitimate reason to take them. His behaviour is so inconsistent from one day to the next. So fidgety, irritable and distracted some day and then so uncharacteristically calm and focused as he was in the debate. I guess they found the right dosage.