We know that Google hates slow pages. Now they have a plan for slow pages. In a recent hangout, John Mueller confirmed this. He said pages that load really slowly may be considered NOT mobile friendly.
Here is the response from John.
There’s one aspect there that does sometimes play a role there in that if you look at things like PageSpeed insights. That we will put together a mobile score for the page.
If a website is technically mobile-friendly in that the UI kind of works on mobile but it really has really bad PageSpeed insights scores then that’s something where we might also think that this probably isn’t a good mobile page.
I am surprised by this. Google said in past that for page speed, they look at desktop page speed only. I guess they could use mobile page speed before displaying not mobile friendly warning. Especially when people are using AMP now. Also, Google shows the same warning to the site owner.