Mobile-first design means building apps and websites to work smoothly on phones...

https://tiny-wiki.win/index.php/How_to_Make_Mobile_Pages_Feel_Smoother_by_Saving_Fractions_of_a_Second

Mobile-first design means building apps and websites to work smoothly on phones first. It keeps things speedy, so pages load fast. Buttons and menus are easy to tap, with plenty of space for your fingers