1. I have never in my life seen evidence of this happening before (enterprise consultant in this field). I am positive in my mind that it was not a case of "page is faster = drop in rankings".
I would be willing to bet that it had a lot more to do with the "rendering content in JS" which has traditionally been a huge issue for search engines (despite what they claim).
But every single time I have seen someone claim this or something like this there has been many other factors at play.
2. It is highly likely that the amount of impact that any given variable (in this case pagespeed) is not the same across the board. In fact it is much more aligned with the kinds of industries you are in and the keywords relating to that search.
As an example page speed is MUCH more likely to be a bigger factor for an e-commerce website and keywords showing any kind of transactional intent than it would be for someone looking for detailed information on a medical condition for example.*
*This isn't a confirmed fact that I know of at all but it seems to be a relatively well established theory in some more advanced SEO circles I believe.