FID (First Input Delay)

First Input Delay (FID) is a Core Web Vitals metric that measures the time between a user’s first interaction and the browser’s response. It assesses how quickly a webpage reacts when users attempt to click a button, link, or form field.

A low FID means the page is responsive, while a high FID indicates delays, often caused by heavy JavaScript execution or blocking resources.

Google uses FID as an SEO ranking factor, as good interactivity improves user engagement and reduces bounce rates.

How FID Works and Thresholds

FID is measured in milliseconds (ms) and represents the waiting time between user input and the browser’s first response.

Google’s FID Thresholds:

  • Good: ≤ 100 ms
  • Needs Improvement: 100 – 300 ms
  • Poor: ≥ 300 ms

FID does not measure content loading speed but rather how fast the page reacts after user input.

Benefits of a Good FID

  1. Smoother user experience: A responsive page prevents user frustration.
  2. Higher SEO ranking: Google prioritizes sites with an FID below 100 ms.
  3. Lower bounce rates: Slow pages cause users to leave quickly.
  4. More intuitive navigation: Faster interactivity improves user engagement.
  5. Better conversion rates: A responsive site boosts actions like sign-ups and purchases.

Causes of High FID & Optimization Strategies

Common Causes:

  1. Too much render-blocking JavaScript
    • Heavy scripts prevent the browser from processing user interactions.
  2. Long main-thread execution
    • If the browser is busy, it cannot respond immediately.
  3. Excessive loading of resources
  4. Unoptimized third-party scripts
    • Ads, analytics, and widgets slow down interactions.

Optimization Solutions:

  1. Reduce JavaScript execution
    • Remove or defer blocking JS.
    • Break up long tasks into smaller async tasks (requestIdleCallback).
  2. Use asynchronous loading (async/defer)
    • Add defer to scripts to prevent them from blocking rendering.
    • Load only essential scripts on first paint.
  3. Optimize main-thread execution
    • Use Web Workers for background script execution.
    • Minimize recalculations and CSS reflows that delay browser response.
  4. Limit third-party scripts
    • Remove unnecessary ads, chatbots, and trackers.
    • Load non-critical scripts after the first page render.
  5. Implement smart caching
    • Enable browser caching for faster repeat visits.
    • Compress CSS and JS files with Gzip or Brotli.

Conclusion

First Input Delay (FID) is a critical metric measuring how quickly a website responds to user interactions. Keeping FID below 100 ms ensures smooth UX and SEO benefits.

By optimizing JavaScript, caching, and third-party scripts, FID can be greatly reduced, improving performance and engagement.

Source : https://web.dev/articles/fid

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