Search engines prioritize mobile users over desktop. This is because the majority of us (56% in 2018) do all of our web browsing through our phones. Mobile data speeds vary wildly by carrier, data plan, coverage area, etc. We all know that not all 4G performs at 4G speeds.
This means that search engines selling advertising based upon the popularity of their search index must find a means by which to deliver the most performant search listings. One of the ways they do this is by using page speed as a ranking metric.
Faster websites rise to the top of search, average and slow sites settle downward. Simply put, faster websites are more mobile friendly. Faster pages will equate to greater exposure to more first-time customers.