Google has been experimenting with their mobile-first indexing for the past year and a half, and they’ve finally made the announcement that they have officially begun migrating websites over to that method, that comply with current best practices. Again, they will ONLY migrate sites which comply with the rules. For example, sites which use responsive web design are well suited for mobile-first.
What is mobile-first indexing?
To provide the best user experience to Google users, the Google spiders or bots will prefer to crawl and recognize mobile versions of pages. More people search using their phones now, so this is the best way to provide the best pages to them. This is not a separate index, it is just a crawl-mobile-pages-first rule for the regular existing index, which was previously crawling desktop versions of pages first.
What does this mean for your website?
You will be notified of your website’s mobile-first indexing being enabled through your Search Console account and you’ll notice a higher crawl rate from the Smartphone Googlebot. If mobile pages are not available, Google will still crawl your website, it just won’t be the ideal and it will affect your ranking within mobile search results, as it has been doing since 2015.
So, the main purpose of this change is not about ranking, but rather a change in the collection of content.
In July 2018, if your site is comparatively slow to load, you are likely to notice a negative affect in rankings for both desktop and mobile searches.
An extra reminder at the end of the announcement restates that ranking is determined by many factors, so while page speed and mobile-friendliness are important, IF the other signals for a particular search determine your content is the most relevant to show the user, it will show.