I'm not sure those things have ever been controlled.
I have been around many many many smart people. So many of them seem naturally gifted. It's effortless how easily they solve things. But if you hang around long enough, it's also impressive when they reveal their magic: they just used so-and-so's technique from some discipline I've never heard of. Oh. Of course! How easy, when you put it that way.
How long did it take them to read, digest, and master all those techniques? Hard to say. Nobody followed them around with a stopwatch 24x7x365 for years.
So, lacking data, I fall back to antecdotes. I personally know very intimately a handful of valedictorian smartest person in the room type people. The kind that everyone knows is just naturally smart. I know their family life intimately. I know that none of it came without enormous time and effort.
We have the testimony of the man himself about the price he paid. I'd be interested in anyone at his level in any endevour who claimed they were just born with it.
I do believe that different people are born with different abilities. But I have seen those "smartest person in the room" people fail elementary math, then spend 100s of hours drilling flash cards to master simple arithmetic tables. I have seen them with reading (I hate the word disabilities) challenges spend 10 years reading 100s of books, then get perfect scores on college entrance exams. I have seen in person some poor child spend 3 hours every single night for a year to master 5 spelling words a week. Just to be average. Then have people think they were born knowing the dictionary.
I believe that if you randomly choose any 20 students from any elementary school in the United States and let me select 10 people from that group, then subject them to what I have seen... those students will graduate high school with perfect grades and nearly perfect standardized test scores. I am certain of it. But I do not condone doing such a thing.
Of course, that doesn't make them Feynman. But I believe Feynman when he says that he was once ordinary and that time and effort transformed him.