What Is INP? A Clear Explanation of Its Meaning, Mechanism, and How to Measure and Improve It

"I want to improve my site's page speed, but I'm not sure what 'INP'—a term I've been hearing a lot lately—actually refers to." "Google Search Console showed that my 'INP is poor,' but I have no idea how to fix it." INP is the starting point for these kinds of concerns. INP (Interaction to Next Paint) is a metric that measures how quickly the screen responds after a user clicks or taps—an indicator of a website's "comfort of operation."

INP became an official Core Web Vitals metric in March 2024 and is positioned as an important factor that also affects search rankings. This article systematically explains, for web managers, the meaning and mechanism of INP, the criteria (thresholds) for judging whether it is good or bad, how to measure your own site's score, and the basic thinking for improving that score—breaking down the technical jargon along the way. By the time you finish reading, you should have a clear understanding of what INP is and what to check on your own site.

What Is INP? A Clear Explanation of Its Meaning

INP stands for "Interaction to Next Paint." In short, it is a metric that measures "the response time from when a user performs an action until the result is shown on the screen." The interaction referred to here means clicking a button, tapping a smartphone screen, keyboard input, and so on.

For example, have you ever pressed the "Add to cart" button on an e-commerce site and, instead of an immediate response, been kept waiting nearly a second? The longer this "wait time from pressing until the response" is, the larger the INP value becomes, and the user feels that "this site is heavy and slow to respond." INP turns this perceived "sluggishness" into a number and serves as a metric for evaluating whether a page's responsiveness is good or bad.

INP is measured in milliseconds (ms = one-thousandth of a second), and the smaller the value, the faster the response and the more comfortable it is judged to be for users. It is distinct from a page's "loading speed"; the defining characteristic is that it looks purely at "how quickly the page responds to operations after it has loaded."

INP Is One of the Three Core Web Vitals Metrics

INP is one of the three metrics in "Core Web Vitals," which Google established to evaluate the usability of a page. Core Web Vitals measures the user experience from three perspectives, each with the following role.

LCP (Largest Contentful Paint): a metric that measures the "loading speed" until a page's main content is displayed
INP (Interaction to Next Paint): a metric that measures the "speed of response (responsiveness)" to operations
CLS (Cumulative Layout Shift): a metric that measures "visual stability"—whether the layout shifts around during loading

While LCP looks at "whether it displays quickly" and CLS at "whether it is stable without shifting," INP is responsible for "whether it responds quickly to operations." The three may appear independent, but it becomes easier to organize them if you understand that each measures the same question—"Is this page pleasant to use?"—from a different angle.

INP Is the Successor to FID (First Input Delay)

Indispensable in discussing INP is its relationship with the previous metric, FID (First Input Delay). In March 2024, INP officially became the metric, replacing FID, which until then had been the Core Web Vitals responsiveness metric.

FID measured only the "wait time (input delay)" of the response to the "first action a user performed." INP, on the other hand, targets not just the first action but every operation during the page visit, and it measures the entire response—including not only the "wait time" but also the "processing time" and the "time until it is shown on screen." In other words, INP has evolved into a stricter and more comprehensive metric that is closer to the actual user experience than FID. For this reason, there are many cases where a page that was "good" under FID is judged "poor" under INP.

How INP Works: Measuring Response Time in Three Phases

Understanding how INP measures response time makes the improvement measures described later easier to grasp. The time from when a user performs an action until the screen is updated is broadly divided into three phases.

Input Delay: the wait time from when a user performs an action until that processing begins. If the browser is busy with other processing (mainly JavaScript), this gets longer.
Processing Time: the time during which the program (event handler) corresponding to the action actually runs and performs the processing. The more complex the processing, the longer it takes.
Presentation Delay: the time until the browser redraws and displays the screen based on the processing result.

INP measures the total time of these three phases as the response time for a single operation. If even one of the three is slow, the overall INP value worsens. Conversely, when your site's INP is poor, separating out "which phase is taking the time" is the first step toward effective improvement.

How the INP Value Is Determined

During a page visit, a user performs many operations. INP observes the response time of all operations that occur during the visit and adopts the value of the slowest operation among them as that page's INP (extreme outliers are excluded). Because even a single slow operation becomes that page's INP, it is a rather strict metric that captures the "moment with the worst experience" as the representative value.

INP Threshold Values: Good, Needs Improvement, Poor

To judge whether your site's INP is good or bad, you need to know the threshold values defined by Google. INP is evaluated in the following three tiers.

Good: 200 milliseconds or less. The page responds almost instantly to operations, and users feel comfortable.
Needs Improvement: more than 200 milliseconds and up to 500 milliseconds. A sluggishness in response can be felt, and it may affect user satisfaction.
Poor: more than 500 milliseconds. There is a clear delay, which easily leads to user frustration and abandonment.

The target to aim for is "200 milliseconds or less (Good)." This criterion is set to maintain a level of responsiveness where users feel that "it responded immediately." Note that because INP varies greatly depending on each user's environment, Google recommends evaluating at the "75th percentile of all users (the value at the 75% mark from the bottom)." This means looking at "whether 75% of users can experience 200 milliseconds or less"—a way of thinking designed to ensure the experience of many users, not just a few high-speed environments.

As a supplementary note, it is also worth keeping in mind that this criterion is evaluated separately for mobile and desktop, and the most demanding combination (in many cases, a long page on a smartphone) tends to become the basis for judgment. INP is currently said to be the Core Web Vitals metric that many sites struggle most to achieve, and it is by no means easy to earn a "Good" rating.

How to Measure INP: Checking Your Site's Score

Because INP cannot be measured without actual user operations, the basic approach is to evaluate it based on "field data (real-user data)." Here we introduce representative measurement methods that web managers can use right away.

Google Search Console

If you are already operating a site, the first thing to check is the "Core Web Vitals" report in Google Search Console. Based on actual user data, you can collectively grasp which pages are judged "poor" or "needs improvement" for INP. It is the ideal starting point when you want an overview of which pages across the whole site have problems.

PageSpeed Insights

PageSpeed Insights is a free tool that lets you check the Core Web Vitals, including INP, for a page simply by entering a specific URL. In addition to real-user field data, it also displays concrete suggestions for improvement, so you can get hints on "how to fix this page." It is suited for when you want to diagnose an individual page in detail.

Chrome UX Report (CrUX) and the Web Vitals Extension

If you want to handle data in more detail, you can use the "Chrome UX Report (CrUX)," in which Google aggregates real-user experiences. Also, when you want to quickly check on hand during development or verification work, using the Chrome browser extension "Web Vitals" lets you check the INP of the page you are viewing in a near-real-time form.

Note that the site diagnostic tool Lighthouse has the convenience of being able to measure simply by loading a page, but because there is no actual user operation, it cannot measure INP itself; instead, it displays a close metric called TBT (Total Blocking Time) as a reference value. Keep in mind that the final evaluation of INP is ultimately determined by real-user field data.

The Basic Thinking for Improving INP

Much of what causes INP to deteriorate is that JavaScript processing occupies the browser's "main thread" (the path of processing responsible for updating the screen and handling operations), hindering the response to user operations. Specialized implementation will end up being delegated to developers, but understanding the direction as a web manager makes it easier to request improvements and judge their effects. The representative ways of thinking about improvement are as follows.

Split up heavy processing: rather than running time-consuming JavaScript processing as one big chunk, divide it into small pieces and create room in between for the browser to respond to operations.
Reduce unnecessary JavaScript: tidy up unused scripts and excessive tags to lighten the load on the main thread.
Review third-party scripts: scripts from external tools such as ads, analytics, and chat often impose a large load, so narrow them down to only what is truly necessary.
Lighten event processing: simplify the contents of the processing that runs on clicks and taps, and reduce the amount of work done at once.

Because the effective measures change depending on which phase (input delay, processing time, or presentation delay) is slow, identifying "which page" and "which operation" is slow with the measurement tools described earlier is, though it may seem like a detour, the shortest route to improvement. Note that simple static sites that use almost no JavaScript tend to have good INP from the start.

How INP Affects SEO and Business

INP is not merely a technical metric; it relates to both search rankings and business outcomes. There are two main reasons.

One is the direct impact on SEO. INP is part of Core Web Vitals and constitutes Google's "page experience" ranking factor. A page with "poor" INP may be at a disadvantage in search rankings compared with a competing page that has high responsiveness. A page that loads quickly, is stable, and responds quickly to operations is positioned as the "excellent page experience" that Google values.

The other is the impact on conversions through the user experience. A site that lags every time it is operated gives users stress and leads to mid-form abandonment and cart abandonment. Improving INP is a measure that works on both the "offensive" and "defensive" fronts: it not only increases search traffic but also captures visiting users and connects them to outcomes without letting them slip away.

Summary: Understand INP and Review Your Site's Responsiveness

INP (Interaction to Next Paint) is a metric representing a page's responsiveness, measuring the speed from when a user performs an action until the screen responds. It is one of the three Core Web Vitals metrics and was officially adopted in March 2024 as the successor to FID. The response time consists of three phases—input delay, processing time, and presentation delay—and the value of the slowest operation during the visit becomes that page's INP.

For evaluation criteria, 200 milliseconds or less is "good," 200 to 500 milliseconds is "needs improvement," and more than 500 milliseconds is "poor," with the target being 200 milliseconds or less. You can check your site's score for free using Google Search Console and PageSpeed Insights. Because much of the improvement hinges on JavaScript optimization, start by using measurement tools to identify the problematic pages and operations.

INP is an indispensable metric for future web operations, relating to both SEO rankings and conversions. First, check your site's INP status in Google Search Console and take the first step by checking whether there are any pages judged "poor."