What is the relationship between INP and the former FID metric?

INP (Interaction to Next Paint) replaced the older FID (First Input Delay) metric as a Core Web Vital in March 2024. FID measured only the delay of the very first interaction, and only up to the start of the response, not the visible result. That was an easy test to pass and said little about the real interaction experience. INP, by contrast, measures all interactions across the entire page visit and reports the worst value (minus statistical outliers).

For optimization this shifts the focus considerably: it is not enough for the page to react on the first click. Opening a menu, filtering a product list or submitting a form must all stay fluid too. To achieve that, we analyze long JavaScript tasks that block the main thread and break them up. More on this topic on the frontend optimization page.