In Google Webmaster Tools, there is a set of graphs that indicate how quickly Googlebot is able to crawl a given website. (To see these graphs, you click on Health > Crawl Stats.) The key metric is shown in the bottom graph, which represents “Time spent downloading a page (in milliseconds)”.
It would be nice if WPT provided something like this.
For example, I’m thinking maybe you could program it as:
Initial Connection (if keep-alive is disabled) + Time to First Byte + Content Download
(For just the initial URL request, obviously.)