Qué es First Input Delay?
Definición Rápida
First Input Delay (FID) mide el tiempo desde la primera interacción del usuario con una página hasta que el navegador puede comenzar a procesar esa interacción.
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 Qué es 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.
Ejemplos Reales
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
Términos Relacionados
Core Web Vitals
Las Core Web Vitals son tres métricas específicas definidas por Google que miden la experiencia real de los usuarios en sitios web: rendimiento de carga (LCP), interactividad (INP) y estabilidad visual (CLS).
Page Speed
La velocidad de página es la medida de cuán rápido se carga y se vuelve interactiva una página web, impactando directamente en la experiencia del usuario y los rankings de búsqueda.
Code Splitting
El code splitting es una técnica de optimización que divide el código JavaScript de una aplicación en partes más pequeñas que se pueden cargar bajo demanda, reduciendo el tamaño inicial del bundle.
LCP (Largest Contentful Paint)
LCP mide el tiempo necesario para que el elemento de contenido visible más grande de una página — como una imagen hero o un título — se renderice completamente.
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