O que é First Input Delay?
Definição Rápida
O First Input Delay (FID) mede o tempo desde a primeira interacção do utilizador com uma página até o browser poder começar a processar essa interacção.
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.
Por Que é 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.
Exemplos Reais
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
Termos Relacionados
Core Web Vitals
As Core Web Vitals são três métricas específicas definidas pela Google que medem a experiência real dos utilizadores nos websites: desempenho de carregamento (LCP), interactividade (INP) e estabilidade visual (CLS).
Page Speed
A velocidade de página é a medida de quão rapidamente uma página web carrega e se torna interactiva, impactando directamente a experiência do utilizador e as classificações nos motores de pesquisa.
Code Splitting
O code splitting é uma técnica de optimização que divide o código JavaScript de uma aplicação em partes menores que podem ser carregadas a pedido, reduzindo o tamanho inicial do bundle.
LCP (Largest Contentful Paint)
O LCP mede o tempo necessário para que o maior elemento de conteúdo visível numa página — como uma imagem hero ou um título — seja totalmente renderizado.
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