Mobile-first design means building a website or app starting with the smallest...

https://wiki-spirit.win/index.php/Why_Do_Users_Judge_a_Brand_by_How_Fast_the_Site_Responds%3F

Mobile-first design means building a website or app starting with the smallest screen—your phone. It focuses on speed, so pages load quickly, and uses big, easy-to-tap buttons for smooth navigation with your fingers