Mobile-first design means building websites or apps starting with the phone...

https://wiki-cafe.win/index.php/Why_Do_Big_Hero_Images_Make_My_Site_Feel_Slow_on_Phones%3F

Mobile-first design means building websites or apps starting with the phone experience. It’s all about fast loading times, easy-to-tap buttons, and simple menus that make getting things done quick and painless on a small screen