Google announced yesterday a new ranking algorithm designed specifically to downgrade the search rankings of really slow mobile pages. Google said "starting in July 2018, page speed will be a ranking factor for mobile searches."
In the blog post, Google said this update "will only affect pages that deliver the slowest experience to users" they also added it "will only affect a small percentage of queries." How small that percentage is, is unclear. But Google has said for a long time only really slow pages have to worry about a downgrade in their rankings for speed. To be clear, there is no ranking boost for being fast, just a downgrade for being really slow.
In April 2010, Google did announce a page speed ranking factor but that only worked on desktop and as we said, Google's mobile search rankings uses desktop speed - which made no sense. We thought, even Google thought, that when the mobile first index goes live that they would not have a page speed factor but we did know they were working on that issue. Well, they were kind of right, because it did start to go live and this mobile speed thing won't be live until July.
Google says you can measure your page speed multiple ways and they are not sharing a specific single metric to know if your site was hit by this algorithm update or not. In fact, Google told me in my big FAQ on the speed update at Search Engine Land that there won't even be a notification in the Google Search Console if you do get hit by this update, that is because this is an algorithmic thing, not a manual action.
Google points webmasters to look at the new PageInsights report over here, also check the Chrome user experience report and use the Lighthouse tool. But again, if you see an unavailable message in the PageInsights report, you will have to use these other metrics.
In any event, when Google announced this, I had several questions, which I published in this FAQ - the biggest question was around AMP.
If a page has an AMP URL but the canonical page, i.e. the mobile URL, is really super slow but the AMP URL is super fast, will Google use the AMP or mobile URL for measuring speed? Logic can say it goes either way, because we know that for indexing and most signals Google will use the mobile page over the AMP URL and heck, even the desktop URL over the AMP URL. So in that case we would assume that the slow canonical mobile page would result in a downgrade. But NO! That is not true, Google told us since the AMP URL is what is being served, even if the mobile URL is super slow, as long as the AMP URL is fast, there is no downgrade. This question alone caused huge confusion and I am glad we got an answer quickly, although I do wish Google would have addressed that question in the original post.
Google's John Mueller said:

(...) In this example, since users from Search would be seeing an AMP page, the speed of the AMP page would be taken into account. However, if a page built with AMP provides a slow experience to users, it may also rank lower in the results.

— John ☆.o(≧▽≦)o.☆ (@JohnMu) January 17, 2018

(...) In this example, since users from Search would be seeing an AMP page, the speed of the AMP page would be taken into account. However, if a page built with AMP provides a slow experience to users, it may also rank lower in the results.
Also, this Speed Update is independent of the mobile first index, if that was not clear. They are two different things:

This is independent.

— John ☆.o(≧▽≦)o.☆ (@JohnMu) January 18, 2018

This is independent.
Overall, this update really is not that big of a deal. I doubt most sites will even notice when it goes live. A page has to be super slow for it to be impacted by this. Heck, even a super fast page won't see any ranking boost. It just hurts pages that are really unbearably slow. How slow exactly? Only Google knows but they won't say.
Here are some, only selected ones, with the original confusion, it is cleared up now.
I even asked a Googler and he didn't know initially:

I'm not really qualified to talk about Search, but I did forward your question!

— Malte Ubl (@cramforce) January 17, 2018

I'm not really qualified to talk about Search, but I did forward your question!
More confusion from Google:

How did you arrive at that? From the blog post: "It applies the same standard to all pages, regardless of the technology used to build the page."

— Ilya Grigorik (@igrigorik) January 17, 2018

How did you arrive at that? From the blog post: "It applies the same standard to all pages, regardless of the technology used to build the page."
So lots of people starting asking the question as well:

Hi Gary. How does AMP fit into this situation? If a site has AMP urls for each of its core pages which load lightning fast (pun intended), then will the algo still use the responsive page speed?? Just want to know more. :) @JohnMu

— Glenn Gabe (@glenngabe) January 17, 2018

Hi Gary. How does AMP fit into this situation? If a site has AMP urls for each of its core pages which load lightning fast (pun intended), then will the algo still use the responsive page speed?? Just want to know more. :) @JohnMu

+1 to this. If the canonical site is slow, but it has an AMP version, which speed index is taking Google under consideration here?

— Óliver Fernández (@oliferna) January 17, 2018

+1 to this. If the canonical site is slow, but it has an AMP version, which speed index is taking Google under consideration here?

By default AMP URLs aren't canonical URLs. So I wouldn't expect them to count towards speed as ranking factor. Nothing changes here!

— Pedro Dias (@pedrodias) January 17, 2018

By default AMP URLs aren't canonical URLs. So I wouldn't expect them to count towards speed as ranking factor. Nothing changes here!

So the AMP users are 'whitelisted'?

— Mayank Parmar 🇮🇳 (@mayank_jee) January 17, 2018

So the AMP users are 'whitelisted'?

Not to be too cynical (which is hard for me) but... this is a great opportunity to sell AMP to those who have thus far avoided it.

— Mordy Oberstein (@MordyOberstein) January 17, 2018

Not to be too cynical (which is hard for me) but... this is a great opportunity to sell AMP to those who have thus far avoided it.

No - Google told us the page that Google shows in their search results is the speed they will use.

— Barry Schwartz (@rustybrick) January 18, 2018

No - Google told us the page that Google shows in their search results is the speed they will use.
Anyway, the answer is AMP for speed will be what Google uses for this algorithm, not your canonical URL because that is what is being served. But for other signals, like content, links, etc, Google will use the canonical mobile URL. Confused? Yea, thought so.
Forum discussion at Twitter & WebmasterWorld.
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