In the hangout from yesterday with Google's John Mueller he said something I never knew before, that pages that load really slowly may be considered NOT mobile friendly.
He said at the 21:39 mark into the video:
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 page and if 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.
Here is the video embed:
Why does this somewhat surprise me? Google has told us that for page speed, they only look at desktop page speed and thus, why in the world would they use that speed for mobile-friendlyness? At least look at the mobile page speed if you are going to use it against the site being mobile-friendly?
What is interesting is that John Mueller said my page speed for the not mobile friendly label I got yesterday was likely the cause:
But the page speed is over 50 for me and others. Plus we have native iOS and Android apps and we are AMP enabled. So shouldn't the mobile friendly work off AMP, since Google serves that page to mobile users?
So the key here is that really slow pages may trigger a page to be not mobile friendly by Google.
Forum discussion at Google+.