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

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

Mobile-first design means building a website or app starting with smartphones in mind. It loads fast, fits your thumb perfectly, and keeps going with simple menus that make finding what you need quick and easy