Why were new metrics introduced?

Other metrics such as First Meaningful Paint (FMP) or Time to Interactive (TTI) didn't tell a story about the first and thus early user engagement. Moreover, third parties that would impact TTI could be loaded without massively impacting the user experience.

At the same time, the web kept growing, impacting UX even more, despite the existing metrics. This is where Cumulative Layout Shift (CLS) comes in, ranking the layout stability while visiting a webpage. First Meaningful Paint wasn't reliable metric as a test could not figure out which elements really were meaningful. This is why FMP got replaced by Largest Contentful Paint.