You may have a slow website, so Google sees that you don't pass this ranking requirement. Websites, regardless of their setup, desktop or mobile, should load in two seconds else the user experience is bad. Mobile speed optimization is a must as Google focuses on mobile devices. What that means is that they will probably bounce, and you've lost them forever. Google has the Chrome browser and Google analytics installed, plus other services to see whether you can attract clicks and keep the visitors engaged with a fast loading website and interesting content. If you don't there's no point in ranking high, is there? Google has introduced the AMP feature to help webmasters build faster websites, plus it provides a speed measuring tool with its tool PageSpeed Insights.
Slower pages are all about bad user experience, meaning if visitors stay, they don't convert well, they don't buy, don't subscribe, don't revisit. On the technical side, they are not friendly to search engine crawlers, which might report errors and not index slow pages. Start with the PageSpeed Insights and work your way into optimizing both website versions.