When it comes to SEO, user satisfaction is most important factor for Google. Closing out slow-loading sites is something most people know well. The world’s fastest cars can go from 0-60 in less than 3 seconds, but the majority of websites don’t load that quickly.
If a page's conversion rate declines, then the total number of users converting goes down even if page traffic remains exactly the same. Conversely, if a page's traffic stays the same but the conversion rate increases, the total number of conversions will go up.
Knowing what we know about human behavior, users will only tolerate low performing applications for so long before abandoning them. Poorly performing sites and applications can also pose real costs for the people who use them and Google understand this very well.
Google will never see your website’s “grade”. Google just looks at how fast your website loads (perception of complete Above-the-fold content load) for real human users, so you don’t have to obsess over that specific score. You have to make sure your website is as fast as it can possibly get.