How does optimization for mobile devices differ?

Mobile optimization is not a side issue but the main benchmark, because Google uses the Mobile-First Index and evaluates the mobile version of your site. Mobile devices also have weaker processors and often less stable networks than desktop PCs, so the same page is noticeably slower there.

In concrete terms: JavaScript bundles must stay small, because parsing on mobile devices takes several times the desktop time. Images are served in responsive sizes so a smartphone never downloads a desktop resolution. Touch interactions must respond without delay to achieve good INP values, and the layout must load without shifts so CLS stays low. That is why we consistently test every optimization under throttled mobile conditions, not just on a fast office machine. More on this in the performance analysis.