I find it funny that Page complained about Gmail taking 600 milliseconds to load back when it was created. It probably takes longer than that these days unless you have a really fast connection.
The article gets the timeline wrong though. This anecdote happened very early in the development of gmail -- it was rewritten several times between then and launch.
I'm sure Google's engineers could get the page load time down if it was a priority. I think at this point everyone is so invested in Gmail they just aren't that worried about losing users because of slower UI load times.