This is not our fault
Many of us use modern devices and fast connections. But many others don't.
Using an iPhone 3GS over a slow connection, you are nearly certain to wait 3 seconds from clicking search to seeing the results.
Basically, Google is saying - this is how long it took us to do the work, any extra delay is not us.
Quality of service (QOS) - Beat us if you can
Have a look at this Underground services performance page. It has links to many documents revealing the QOS. Transparency comes to mind here.
You can see it as bragging, or you can see it as: "This is how well we do".
Had I written a search engine faster than Google, I sure am to put that figure on my page. I suspect other search engine don't because that will reveal Google's superiority. Which is probably exactly why Google includes it - Beat us if you can.