In a nutshell, mobile first indexing means that one of the largest factors in ranking your website in Google search will now be if it's optimized for mobile devices. Not just responsive. Not backwards compatible. Not "looks pretty good on a phone."
Optimized.
The difference is clear. Your website will load faster out of the box than any of the other major site builders.
When you open a website in Google Chrome, you can test the site's speed index against Google's own metircs. Here's an article explaining how to open Google's auditing tools in Chrome if you want to try this yourself.
Websites are graded on a scale of 0% to 100%. A perfect score is almost impossible on a modern business website.
A score of 90% or better will CRUSH your competion in speed indexing.
Take a look for yourself:
Our competitors all build their homepages using their own website builders. How do they stack up on Google Speed Indexing?
Here are the three top selling web builders in the country:
If they weren't nationally known brands... they wouldn't rank on Google at all.