Reading speed.
It took me a while to realize it because the only time I have a direct comparison is when someone is reading something over my shoulder at the computer, but whenever I read something with anyone other than my parents I spend most of the time waiting for the other person to finish. Sometimes people don't believe it, like someone shows me an article, I scroll down, and they get mad because they think there is no way I had actually read it because they were still on the first paragraph. At first I thought it was just because most people in my grade school never read anything outside of school, but eventually enough people whose intelligence I respected commented on it some way or another for me to realize it is unusual. Now I am in my 30s and work with some very smart people, and it still happens if I read something with someone else.
To be clear, this is not "speed reading," where you just skim the text looking for keywords and therefore do not comprehend most of it. I actually read, going over some things multiple times to understand better, and still come out far ahead.
My mom taught me to read when I was 3 or 4 years old, so I was shocked to learn that most people don't learn to read until first grade, in a way that hamstrings them for life without anyone even realizing it. From what I understand, most people learn to read first by reading out loud, then by moving their lips, then by subvocalizing, then finally by imagining a voice in their head speaking the words to them. This is a severe limitation because it effectively means reading speed cannot exceed speaking speed. Not necessarily one's own speaking speed, but anyone's: for example, Busdriver is probably the fastest rapper in the world, and he is almost incomprehensible at fewer than 300 words per minute. I read about three times that fast because I never had to unlearn a pointless intermediate auditory processing step. Text becomes information directly, with no thinking about how the words sound.
To put that speed in perspective, if I start reading a Wheel of Time book in the morning and read all day, I finish it by bedtime. The entire series takes two weeks. The Dune books are shorter but denser, and therefore also take about a day per book. Harry Potter is much easier: I bought the last three Harry Potter books when they came out at midnight and finished by breakfast. My family is the same way, so this is normal for me.
Reading efficiently is such a huge advantage in so many ways that I consider it child neglect to teach children to read the usual way. Imagine how different society would look if everyone could keep up with the news, absorb three times as much information while wasting time on Wikipedia or Reddit, never worry about time limits on tests, find words without ctrl+f, etc.