A couple of weeks ago, I started receiving notifications from Google that: “Mobile-first indexing” was enabled for many of our websites. According to Google, the background of this is: “Mobile-first indexing means that Googlebot will now use the mobile version of your site for indexing and ranking, to better help our (primarily mobile) users find what they’re looking for. Google’s crawling, indexing, and ranking systems have historically used the desktop version of your site’s content, which can cause issues for mobile searchers when the desktop version differs from the mobile version. Our analysis indicates that the mobile and desktop versions of your site are comparable.”
In short, if your desktop version and smartphone version of your website are comparable, then you should not see any effect on your traffic. However, if you are producing 2 separate websites, one for each device, then your traffic could take a hit. The Speed and Mobile-Friendliness of your site is not what attributes to mobile-first indexing. This is simply a content issue.