Edit: in the case of Google and Amazon, I can believe that being slow will cause users to defect to other services. I don't believe that anybody will not bother to read documentation because it takes a second to load.
Edit2: If this is true, can anybody explain why users behave in this seemingly bizarre way? Do you give up on pages after 500ms? Have you seen anybody else do that? What is going on?
By analogy, to get into a different mindset, think about channel-surfing on the TV. If other channels show a picture in 0.2s, and as you flip around there's a channel that takes 0.8s to show a picture, are you more or less likely to surf past the slow channel?
Thinking about it I can well believe that if you want people to stay on your site and click around, probably because you want to show them ads or products, a small delay will impact on the number of clicks. I would imagine that's not the motivation for most github pages sites though.