Cos'è First Input Delay?
Definizione Rapida
Il First Input Delay (FID) misura il tempo dalla prima interazione di un utente con una pagina fino a quando il browser può iniziare a elaborare quella interazione.
First Input Delay captures the delay users experience when they try to interact with a page that's still loading. If a user clicks a button but the browser is busy parsing JavaScript, there's a gap between the click and the response. That gap is FID.
FID is important because it measures real user experience, not lab data. It can only be measured in the field because it depends on actual user behavior. Google's Web Vitals program considers FID good if it's under 100ms, needs improvement between 100-300ms, and poor above 300ms.
The primary cause of high FID is heavy JavaScript execution during page load. When the browser's main thread is busy parsing and executing JavaScript, it can't respond to user interactions. Third-party scripts (analytics, ads, chat widgets) are common culprits.
Note: Google has replaced FID with Interaction to Next Paint (INP) as a Core Web Vital, which measures all interactions throughout the page lifecycle, not just the first one. However, FID remains a useful concept for understanding initial page interactivity.
Reducing FID involves minimizing and deferring JavaScript, breaking up long tasks, using web workers for heavy computation, and implementing code splitting to reduce the amount of JavaScript that runs during initial page load.
Perché è Importante
Users notice delays as short as 100ms. When they click a button and nothing happens, they click again, or they leave. High FID creates a perception that your website is broken or slow, even if it eventually loads correctly.
Since FID (and its successor INP) is a Core Web Vital, it directly affects Google search rankings. Sites with poor interactivity metrics may rank lower than faster competitors.
Esempi Pratici
A news website had 500ms FID because their ad scripts were monopolizing the main thread during load — deferring these scripts reduced FID to 50ms
An e-commerce site's product page had 300ms FID from a heavy JavaScript bundle — code splitting and lazy loading reduced it to under 80ms
A marketing site removed 3 third-party chat and analytics scripts and improved FID from 250ms to 30ms, with minimal impact on functionality
A web app moved their data processing logic to a Web Worker, freeing up the main thread and reducing FID from 400ms to 60ms on mobile devices
Termini Correlati
Core Web Vitals
Le Core Web Vitals sono tre metriche specifiche definite da Google che misurano la reale esperienza degli utenti sui siti: prestazioni di caricamento (LCP), interattività (INP) e stabilità visiva (CLS).
Page Speed
La page speed è la misura di quanto rapidamente una pagina web si carica e diventa interattiva, con impatto diretto sull'esperienza utente e sui ranking dei motori di ricerca.
Code Splitting
Il code splitting è una tecnica di ottimizzazione che divide il codice JavaScript di un'applicazione in parti più piccole che possono essere caricate su richiesta, riducendo la dimensione iniziale del bundle.
LCP (Largest Contentful Paint)
LCP misura il tempo necessario perché il più grande elemento di contenuto visibile su una pagina — come un'immagine hero o un titolo — venga renderizzato completamente.
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