I think there was a time where Biden was very bright, fast, a deep/complex thinker (although he was a corrupt slime ball back then too).
But he had several brain surgeries that deliberately do not get talked about in our media (at least two).
You can “literally” hear the difference in speech before and post surgeries.
The point being that his obvious body language, outbursts when confronted, are a result of his inability to control the signals or feelings he has.
We had a feeble mind running this country for 4 years.
While "book smarts" may not equal intelligence, I don't think he was ever "sharp as a tack". Slick? Yes. Bright? No.