Introduction
Over the past few years, Google’s Core Web Vitals have redefined what makes a website “fast” in the eyes of both users and search engines. Initially, metrics such as Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS) dominated the optimization landscape. However, 2025 brought a major shift: Interaction to Next Paint (INP) officially replaced FID as the primary responsiveness metric.
Why does this matter? Because INP captures how responsive your site feels during a full user session. It doesn’t just measure the first interaction — it evaluates every tap, click, and keypress. For businesses, developers, and SEO professionals, this change means one thing: interactivity is now central to both user experience and search visibility.
In this blog, we’ll explore the key differences between INP, FID, and TBT, why Google made the switch, and which of these metrics truly matters most for your SEO strategy in 2025.
Understanding the Metrics — INP, FID, and TBT
What Is INP (Interaction to Next Paint)?
INP (Interaction to Next Paint) measures the time between a user’s interaction (such as clicking a button or typing) and the next visual change on the screen. Unlike FID, which focuses only on the delay before processing begins, INP captures the entire lifecycle of the interaction — from the input event to the moment the interface visually responds.
In simple terms, INP reflects how fast a site feels when you interact with it.
Good INP: ≤ 200 milliseconds
Needs Improvement: 200–500 milliseconds
Poor: > 500 milliseconds
A low INP means that every action on the page — like expanding menus, submitting forms, or navigating — feels instantaneous.
What Is FID (First Input Delay)?
FID (First Input Delay) measures how long it takes for a site to respond to the first user interaction after it becomes interactive. Introduced in 2020 as one of the original Core Web Vitals, FID helped developers gauge responsiveness but had a critical limitation: it only looked at the first interaction.
For example, if a user clicked once on a menu and the page responded quickly, FID was considered good — even if later interactions were laggy. This gave an incomplete picture of user experience.
Why FID Became Outdated:
- It ignored interactions beyond the first one.
- It couldn’t detect input delays caused by long-running tasks later in the session.
- As web apps became more dynamic, FID’s narrow scope no longer represented real user behavior.
What Is TBT (Total Blocking Time)?
TBT (Total Blocking Time) is a lab metric that quantifies how long the main thread is blocked by long tasks (anything exceeding 50 milliseconds). It acts as a bridge between synthetic testing and field performance.
Unlike INP and FID, TBT doesn’t rely on real-user interactions — it’s measured in controlled lab tests like Lighthouse. However, it correlates strongly with INP: a lower TBT often leads to better INP scores.
TBT remains critical during development because it highlights JavaScript-heavy operations that could delay input handling in the field.
Why INP Replaced FID in 2025
When Google introduced INP as an experimental metric in 2022, data from the Chrome User Experience Report (CrUX) revealed that FID failed to capture many user frustrations. People often experienced laggy scrolling, delayed clicks, or unresponsive dropdowns — none of which FID measured accurately.
By 2024, Google confirmed that INP would officially replace FID in March 2025.
Core Reasons for the Change
- Comprehensive Measurement:
INP measures all interactions, not just the first one. - Realistic Representation:
INP aligns with how users actually perceive speed — consistent responsiveness matters more than one quick click. - Improved Diagnostic Value:
INP pinpoints whether issues arise from input delay, processing time, or rendering bottlenecks. - Better Correlation with User Satisfaction:
Studies showed that sites with a “Good” INP had higher engagement, lower bounce rates, and better retention than those focusing only on FID.
Impact of the Transition
For developers, the shift required a mindset change. Optimizing for INP means improving responsiveness throughout the page lifecycle. For marketers and SEO experts, INP became a core ranking factor, influencing both organic visibility and user experience metrics like dwell time.
Comparing INP, FID, and TBT — Core Differences
| Metric | Measures | Data Source | Strength | Limitation |
|---|---|---|---|---|
| INP | Time between interaction and next paint (full session) | Field data (real users) | Most accurate measure of responsiveness | Harder to test in labs |
| FID | Delay before first input processing starts | Field data (first interaction) | Simple to track | Only measures one event |
| TBT | Total time main thread is blocked by long tasks | Lab data (synthetic) | Great for debugging | Not real-user data |
INP vs FID
INP evaluates every interaction — buttons, dropdowns, scrolls — whereas FID measures only the first. For single-page apps or content-heavy platforms, INP provides a far more reliable indicator of true responsiveness.
INP vs TBT
While TBT doesn’t measure real-user experience directly, it helps developers identify blocking code that will affect INP in the field. In other words, you optimize TBT to improve INP.
FID vs TBT
FID and TBT once worked hand-in-hand: FID for field data, TBT for lab correlation. Now, INP and TBT form the new performance pair.
How INP, FID, and TBT Affect Core Web Vitals and SEO
Google’s goal with Core Web Vitals is simple — reward websites that feel fast to users. Interactivity has always been crucial, but INP’s arrival changes how we measure it.
INP’s Influence on SEO
- Ranking Factor: INP is now a confirmed component of Core Web Vitals assessments.
- User Engagement: Faster interactions increase dwell time and conversion rates.
- Behavioral Signals: Responsive interfaces reduce pogo-sticking (users bouncing back to results).
A site with a “Good” INP consistently provides instant feedback to user inputs, which Google interprets as a positive user experience.
FID’s Diminishing Role
Although FID is officially deprecated, Google still references it historically in older reports. Developers maintaining legacy analytics may continue tracking it for comparison but should transition to INP for future readiness.
TBT’s Continued Importance
Even in 2025, TBT remains invaluable during development. It provides actionable feedback in synthetic environments where INP can’t be measured. By reducing TBT, developers indirectly improve INP.
For SEO strategists, this means lab optimization (TBT) and field validation (INP) now work hand in hand.
Practical Insights — Optimizing for Each Metric
Optimizing INP
- Reduce Main-Thread Work:
Avoid heavy JavaScript that blocks user input. Split long tasks into smaller asynchronous chunks. - Minimize Event Handler Delays:
Simplify logic in click, scroll, and input events to ensure quick execution. - Defer Non-Critical Scripts:
Load analytics, ads, and widgets only after the main content is interactive. - Use Real-User Monitoring (RUM):
Measure INP in the field to identify device-specific slowdowns. - Monitor Long Tasks Continuously:
Set performance budgets to prevent regressions during new releases.
Improving FID (Legacy)
If your analytics still use FID, the same principles apply: avoid heavy main-thread blocking during initial page load. Quick first interaction still contributes to positive perceived performance, even if the metric is deprecated.
Lowering TBT
- Eliminate Long Tasks (>50 ms).
Every long task delays browser response. - Reduce JavaScript Bundles.
Lighter payloads reduce parsing and execution delays. - Optimize Third-Party Scripts.
Audit all external resources — many block rendering unnecessarily. - Monitor TBT in Lighthouse.
Aim for <200 ms for optimal field INP performance.
When you improve TBT in the lab, you generally improve INP in real life.
Future of Web Performance Metrics
As web experiences evolve, Google’s Core Web Vitals are becoming more user-centric and behavior-aware. The shift from FID to INP symbolizes this transition from technical measurements to experience-driven metrics.
Predictions Beyond 2025
- AI-Assisted Performance Analysis:
Future tools will predict responsiveness issues before deployment using AI-based modeling. - More Granular Field Data:
Developers will have access to device-specific INP scores, showing performance differences across hardware tiers. - Expansion Beyond Core Web Vitals:
Additional UX signals — like input predictability or animation stability — may become ranking considerations. - User Perception Becomes the Benchmark:
Metrics will increasingly align with how users feel performance, not just what the browser measures.
INP is likely to remain central to this evolution because it reflects genuine interaction smoothness across all devices and sessions.
Conclusion
By 2025, INP has firmly taken the lead as the key indicator of a website’s responsiveness and user experience.
FID served its purpose but was limited to a single interaction.
TBT remains an essential development tool, helping diagnose blocking operations that degrade performance.
But INP is now the metric that truly matters — it captures the responsiveness users actually experience throughout their journey.
For developers, optimizing INP means refining how JavaScript executes, deferring unnecessary work, and continuously measuring field data. For marketers and SEO professionals, it means prioritizing sites that feel as fast as they look.
INP vs FID vs TBT: Which Core Web Vital Matters Most for SEO in 2025