What Is First Input Delay (FID) and Why It Matters for UX & SEO
FID is one of Google's Core Web Vitals and directly impacts your website’s user experience and SEO performance. A low FID means users can interact with your site quickly. A high FID creates friction. It often results from heavy JavaScript execution blocking the browser from responding to user inputs in real-time. Google recommends an FID under 100 milliseconds to ensure an optimal experience. While FID is being replaced by INP (Interaction to Next Paint) as the primary responsiveness metric by March 2024, understanding and optimizing FID remains essential for legacy performance and overall responsiveness benchmarks.
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Use Cases
Reducing heavy main-thread tasks minimizes FID, improving first-click responsiveness and making your site feel faster.
Shoppers demand speed. Optimizing FID can improve click-to-cart responsiveness and increase your checkout flow success rate.
Google uses FID as a ranking signal. Faster interaction = stronger SEO performance = more organic traffic.
Improving Mobile Experience
Frequently Asked Questions
What causes high FID?
Most commonly, FID issues are caused by long-running JavaScript blocking the main thread, preventing the browser from responding to user interactions promptly.
Is FID still important after INP replaces it?
Yes — while INP will replace FID in 2024, understanding and optimizing FID helps address the same root issues (input delay), ensuring smoother page interaction.
What’s a good FID score?
According to Google, an FID under 100 ms is considered 'Good,' between 100–300 ms is 'Needs Improvement,' and over 300 ms is 'Poor'. Aim for under 100 ms.
How can I measure FID?
FID measures responsiveness—how fast the page reacts to the first user action. LCP (Largest Contentful Paint) measures loading speed—how fast key page content appears.
What’s the difference between FID and LCP?
Prioritize main-thread performance: defer non-critical JS, use code-splitting, reduce third-party scripts, minimize blocking tasks, and use browser performance APIs effectively.
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