First Input Delay (FID)

Core Web Vitals Interactivity Measurement

FID is one of Google’s three Core Web Vitals metrics that affects search rankings and measures real user experience. Google considers FID scores under 100 milliseconds as “Good,” between 100-300 milliseconds as “Needs Improvement,” and over 300 milliseconds as “Poor.” Unlike other metrics that can be measured in lab environments, FID requires real user interactions, making it a field-only metric that reflects actual user experience.

JavaScript Performance Impact

FID problems typically occur when the browser’s main thread is busy executing JavaScript code. When users try to click a button, tap a link, or interact with a form while JavaScript is running, the browser cannot respond immediately. Common causes include large JavaScript bundles, third-party scripts like analytics or chat widgets, and inefficient code that blocks the main thread during critical loading periods.

User Experience and Engagement

Poor FID creates frustrating user experiences where clicks and taps feel unresponsive. Users may click multiple times thinking their first interaction didn’t register, or they may abandon the page entirely. Good FID ensures that when users try to interact with your content, the page responds immediately, creating a smooth and engaging experience that encourages continued interaction.

Mobile Device Considerations

FID issues are often more pronounced on mobile devices due to less powerful processors and limited resources. Mobile users are also more likely to interact quickly with pages, making responsive interactivity crucial. JavaScript that runs smoothly on desktop computers may cause significant FID problems on mobile devices, requiring specific optimization for mobile performance.

WordPress FID Challenges

WordPress sites often struggle with FID due to multiple JavaScript files from plugins, themes, and third-party integrations. Common FID problems include heavy page builders, numerous plugins loading JavaScript, analytics scripts, social media widgets, and advertising code. WordPress optimization for FID typically involves script optimization, plugin management, and careful timing of when JavaScript loads and executes.

“FID problems only affect JavaScript-heavy websites”

Even simple websites can have FID issues if they load third-party scripts, analytics code, or advertising that blocks the main thread during initial page loading.

“Removing all JavaScript eliminates FID issues”

While removing JavaScript would improve FID, modern websites need JavaScript for functionality. The goal is to optimize JavaScript loading and execution, not eliminate it entirely.

“FID is the same as page load speed”

FID specifically measures interactivity responsiveness, not loading speed. A page can load quickly but still have poor FID if JavaScript blocks interactions.

Google Resources & Guides:

• Web.dev FID Guide
• Web.dev JavaScript Performance
• Web.dev JavaScript Code Splitting Guide
• Web.dev Third-Party Script Optimization Guide
• Web.dev FID Optimization

FID Testing and Monitoring Tools:

• Google PageSpeed Insights
• Chrome User Experience Report