Well to be extremely specific, mobile first has something to do with speed, both styling speed and file size can be affected if you don’t go with a mobile first approach, as the mobile designs normally make wider use of browser default styles (e.g. divs displayed as blocks that might be switched to any other layout in wider screens). That being said, difference is negligible, specially after minifying and Gzipping.
Mobile first is a way of thinking the design and development of your websites.
And considering the statistics, I’d say not only it’s still relevant, but it’s getting more relevant by the minute, as the tendency clearly shows an ever increasing percentage of users accessing from mobile devices compared to desktop / laptops.
