Mobile-first design means building apps and websites with phones in mind first....

https://felixssuperperspective.brightsora.com/posts/why-do-people-abandon-a-site-when-it-loads-slowly-on-mobile

Mobile-first design means building apps and websites with phones in mind first. This makes pages load faster, so you don’t have to wait. Buttons and menus are bigger and easy to tap with your finger