When developing a web application it is essential to respect a certain level of requirement on the UX. This practice allows us to satisfy the user but also to ensure that he enjoys using our application.

In 2020, Google announced that the following year it would integrate three new signals into its performance and metrics tools to characterize and measure this user experience.

These signals are grouped under the name of Core Web Vitals and are the following:

• LCP (Largest Contentful Paint)
• CLS (Cumulative Layout Shift)
• FID (First Input Delay)

But what do these metrics actually mean? How can we debug and optimize them and make our application user-friendly?

Through this talk, I will answer these questions and also present a fourth signal still under experimentation: INP (Interaction to Next Point)