Wat is First Input Delay?
Snelle definitie
First Input Delay (FID) meet de tijd van de eerste interactie van een gebruiker met een pagina totdat de browser die interactie kan beginnen te verwerken.
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.
Waarom het belangrijk is
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.
Praktijkvoorbeelden
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
Gerelateerde termen
Core Web Vitals
Core Web Vitals zijn drie specifieke statistieken van Google die de werkelijke gebruikerservaring op websites meten: laadprestaties (LCP), interactiviteit (INP) en visuele stabiliteit (CLS).
Page Speed
Paginasnelheid is de maatstaf voor hoe snel een webpagina laadt en interactief wordt, met directe invloed op de gebruikerservaring en zoekmachinerankings.
Code Splitting
Code splitting is een optimalisatietechniek die de JavaScript-code van een applicatie opsplitst in kleinere stukken die op verzoek kunnen worden geladen, waardoor de initiële bundelgrootte wordt verkleind.
LCP (Largest Contentful Paint)
LCP meet de tijd die nodig is voor het grootste zichtbare contentelement op een pagina, zoals een hero-afbeelding of een koptekst, om volledig te worden weergegeven.
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