It is official, from July 2018, page load speed will become a ranking factor in mobile search. This change, called the “Speed Update” will start off with only a small percentage of search queries. However, as with other Google Search changes, will probably hit the full index soon after. The good news is that the update is meant to penalize the slowest sites, and not to boost the fastest:
The official statement by Zhiheng Wang and Doantam Phan from Google:
The “Speed Update,” as we’re calling it, will only affect pages that deliver the slowest experience to users and will only affect a small percentage of queries. It applies the same standard to all pages, regardless of the technology used to build the page. The intent of the search query is still a very strong signal, so a slow page may still rank highly if it has great, relevant content.
The recommendation is to use the PageSpeed report or a third-party tool to measure the page load speed and see if the update will affect your site. The speed of page loads on mobile devices is more important than on desktops, which means you can expect Google to slowly increase the importance of the ranking factor.
Generally, Google updates that penalize sites are harsher than the boost you get from a positive ranking signal, which is why it is so important that webmasters measure and address any page speed issues on their mobile pages.