Why mobile first is outdated

just a clarification: for me “mobile first” is not just a design or a screen thing, it’s one technical way to implement CSS media query and load only the elements you need, the internet speed is often better on Desktop than Mobile 3G. So why load a BIG 2000px picture on a little 320px screen and loose precious loading time when we know that the user experience is impacted from less than 0.3 seconds of loading? ok, today they are 4G and retina and @4x screen, but they still old fashion device.