In short, the webmaster said, it is frustrating to land on a mobile friendly web page that takes forever to load. The feedback is around Google still using many desktop signals for mobile ranking - including how fast a desktop page loads and not counting the speed of the mobile page.
Gary said to that "we have nothing to announce, but if you think about it, there's no reason for us to stop improving our MF algos." As we reported months ago, Google is testing a slow label on mobile results - so Google seems to be testing determine the page speed of mobile sites. But for that to be incorporated into the overall mobile algorithm - that can take time.
I'd be surprised if Google didn't incorporate some of these mobile only factors such as page speed, content on the mobile page, etc etc sooner than later.
Forum discussion at Twitter.