Every extra second a page takes to load costs a little of the visitor’s patience. Speed is not a technical detail; it is the shop door.
Three measures, three questions
LCP: When did the main content appear?
INP: How quickly did the site respond to a tap?
CLS: Did the content jump while reading?
Where to look first
Large images, unnecessary third-party scripts and late-loading fonts cause most of the problems on most sites.