The reason this has come up is that someone found a section about it in their help pages, but it had not been implemented then, and they announced it around various forums. I was told it was going to be added to the QS algo, so the information was placed on the help pages, but they decided to push back the launch and forgot to remove this bit.
https://adwords.google.com/support/?fulldump=1 (search for "page loading times")
After it was out, they had to push the launch a bit early.
It makes sense anyway, a slow page load time is a bad user experience.