Like watching someone take 5 seconds per keystroke because they have to hunt and peck on the keyboard while slowly and deliberately sounding out the letters they're typing?
You know that feeling of wanting to just push them out of the way because you could get whatever completely done by the time they finish the next step in the process?
You ever felt like grabbing someone's face and just point blank ask them why they don't "get it"? Because it's the simplest thing in the world to you?
Now imagine your entire life was that. Of essentially having to wait for people to catch up to where you were. Couple that with the nagging thought in the back of your mind that you could be the one that's wrong because so many people are on the other side of the issue. What do they see that you don't? And then when they finally do catch up, treat it like it was the most obvious thing ever.
I can see how that would wear on people. How that would make them depressed, how they would be difficult to relate to, how it would be difficult to communicate with them.
Sometimes I feel saying that intelligent people have difficulties relating to people or communicating with others is akin to saying it's difficult to run as fast as Usain Bolt. It's like "No shit, but it's not a problem on Bolt's end".