Adam Bradley asked Google's John Mueller on Google+ how does Google measure the "time spent downloading a page" metric found within the Crawl Stats section of Google Search Console.
John Mueller said it has nothing to do with how long the page takes to render, but rather a simple HTTP request time. Basically, how long it takes GoogleBot to complete the HTTP request.
John wrote, "that's just time to complete a HTTP request, nothing related to rendering the page itself."
So this does not measure page speed at all.
Forum discussion at Google+.