Definition
FID (First Input Delay) is a Core Web Vital metric that measures interactivity by quantifying the time between a user’s first interaction with a page (such as clicking a button or link) and the browser’s ability to respond to that interaction. As a user-centric performance measurement, FID evaluates how responsive a page feels during the critical initial engagement period when users form their impression of a site’s performance. FID is measured in milliseconds, with Google classifying performance as “good” when the delay is less than 100 milliseconds.
Unlike metrics that focus on loading or visual rendering, FID specifically targets interaction responsiveness during the page load process. It measures actual user experiences in the field rather than simulated interactions in lab environments. FID captures delays caused by the browser’s main thread being occupied with parsing and executing JavaScript, which prevents it from responding promptly to user interactions.
Key characteristics of FID include:
- Measurement of the delay between a user’s first interaction and the browser’s response
- Classification thresholds of “good” (≤ 100ms), “needs improvement” (≤ 300ms), and “poor” (> 300ms)
- Status as one of the three Core Web Vitals and a confirmed Google ranking factor until March 2024
- Evaluation based on the 75th percentile of user experiences
- Capture of only the input delay, not the processing time or rendering time after the browser begins handling the interaction
- Collection only through field data (real user monitoring) as it requires actual user interaction
- Particular focus on the period when a page is still loading and JavaScript execution may block the main thread
- Measurement only of the first interaction, ignoring subsequent interactions regardless of their delay
- Scheduled replacement by Interaction to Next Paint (INP) in March 2024
History of FID
FID has evolved alongside the broader development of user-centric performance metrics:
2018-2019: Google’s Chrome team begins developing interaction-based metrics to better measure real-world responsiveness beyond loading speed.
2020 (May): Google officially introduces FID as part of the Core Web Vitals initiative, establishing it as the primary metric for evaluating interactivity performance.
2020 (November): Google confirms that FID, along with other Core Web Vitals, will become ranking factors in 2021 as part of the Page Experience update.
2021 (June-August): The Page Experience Update rolls out, officially making FID a ranking factor in Google’s search algorithm.
2022: Google begins acknowledging limitations in FID, particularly its focus only on first interaction and inability to capture the full processing time of interactions.
2023 (May): Google announces plans to replace FID with Interaction to Next Paint (INP) in March 2024 to provide a more comprehensive measurement of overall page responsiveness throughout the user journey.
2024 (March): FID is scheduled to be officially replaced by INP as a Core Web Vital, ending its status as an official ranking factor.
Types of FID Measurements
FID captures various user interaction types and scenarios:
Click Interactions: Measurement of delays when users click buttons, links, or other interactive elements.
Tap Interactions: Touch-based interactions on mobile devices, which often show higher FID values due to device limitations.
Keyboard Input Delays: Time until the browser processes key presses, particularly relevant for search boxes and forms.
JavaScript-Heavy Page FID: Particularly problematic on pages with large JavaScript bundles that monopolize the main thread during initial loading.
Third-Party Script Impact: FID measurements affected by advertising scripts, analytics, or other third-party code that blocks the main thread.
Long Tasks FID: Delays caused by JavaScript execution tasks that exceed 50ms and block user interaction.
CPU-Limited Device FID: Significantly higher delays experienced on lower-powered devices like budget smartphones.
Network-Constrained FID: Interaction delays influenced by slow resource loading that triggers additional processing during interaction attempts.
Framework Hydration FID: Delays specific to JavaScript frameworks that use hydration to make server-rendered content interactive.
Transition FID: Measurements taken during page transitions in single-page applications when new content is being processed.
Importance in Modern SEO
FID has been significant in contemporary SEO practice for several compelling reasons:
As an official Google ranking factor until March 2024, FID directly influences search visibility as part of the Core Web Vitals. While content relevance remains paramount, meeting the “good” threshold for FID provides competitive advantages in search rankings, particularly when content quality is similar between competing pages.
Beyond search rankings, FID significantly impacts user experience perception, particularly for first-time visitors forming initial impressions. Research consistently shows that delays in interactivity lead to user frustration and increased bounce rates. Google’s studies indicate that when FID improves from “poor” to “good,” the probability of users abandoning pages decreases by approximately 24%. These behavioral signals indirectly reinforce ranking benefits by improving engagement metrics.
For mobile users—who often face device processing limitations and inconsistent network conditions—FID performance is particularly crucial. With mobile-first indexing now standard, ensuring responsive interactions on mobile devices directly impacts overall search visibility.
The focus on FID has driven significant improvements in JavaScript delivery and execution practices. Techniques like code splitting, progressive hydration, worker threads, and optimized event handling have become standard largely due to their impact on interaction responsiveness. This evolution has elevated technical SEO’s role in development decisions around JavaScript architecture and delivery.
For interactive websites like e-commerce platforms, social media applications, and form-heavy services, FID optimization directly impacts conversion rates. Studies show that improved interactivity correlates with higher form completion rates and transaction success, creating business benefits beyond SEO advantages.
While being replaced by INP in March 2024, the lessons learned from FID optimization remain valuable. The same techniques that improve FID generally benefit INP, and the organizational focus on interactivity that FID helped establish will continue to provide value as metrics evolve. Understanding FID also provides historical context for interpreting performance data that spans the transition to INP.
As user expectations for web responsiveness continue to increase across devices and contexts, maintaining competitive interactivity metrics requires ongoing optimization rather than one-time fixes. This necessity has led many organizations to implement performance budgets and automated monitoring systems specifically targeting interaction responsiveness, institutionalizing performance as a continuous consideration.Retry