While "book smarts" may not equal intelligence, I don't think he was ever "sharp as a tack". Slick? Yes. Bright? No.
What I’m saying is far from scientific, rather mere observation
I am basing it on his earlier speeches, pre-brain surgeries.
He was definitely faster, the sentences were more complex as were the ideas expressed.
Biden and those dependent on him, didn’t want his brain surgeries to get in the way of his political future / survival. However, the way he performed as President proves that these sort of issues (physical and mental health) SHOULD be considered, it should have been an issue. Slow-Joe was not firing on all cylinders from day one and it was obvious.
The MSM which pressured Trump to be mentally evaluated in his first term, gave Biden a free pass even though in addition to two brain surgeries and an OBVIOUS mental degradation, he was also one of the oldest Presidents to ever have served.
Had the media been doing their job and not just playing partisan politics, there would have been calls for neutral / unbiased testing, and he would have failed. He should have never even been the Democratic parties choice. Him being the candidate is very telling about the political process.
Biden was not able to fulfill his duties as President.