INP in WordPress instead of FID: how to check your site’s responsiveness

The quickest way to check INP on a WordPress site is PageSpeed Insights. The tool shows field data from the CrUX report, which means measurements from real Chrome users. Don’t put this off: according to the Chrome team’s announcement of the change, INP replaces FID in Core Web Vitals as of March 12, 2024. Below I explain what the new metric is, where to measure it, how to read the rating and what to do when the score is poor.

What is Interaction to Next Paint and why did it replace FID?

Interaction to Next Paint measures the time from a user interaction (a click, a tap on the screen, a key press) until the next frame is displayed. Put simply: how quickly the page responds throughout the whole visit, not just at the start. FID only looked at the delay of the first interaction. And that was the catch. A site could have a nice score and still stutter when adding a product to the cart or opening the menu.

The change didn’t come out of nowhere. As the web.dev blog post on the INP launch describes, the metric appeared as experimental in May 2022, then became a pending metric, and on March 12, 2024 it became a permanent part of Core Web Vitals. FID is dropped from the Web Vitals program, and its documentation now points to materials about INP.

What changes in Core Web Vitals 2024 for WordPress sites?

Core Web Vitals now consist of three metrics: LCP (how fast the largest element is displayed), CLS (layout stability) and INP in place of FID. The rating doesn’t come from a single test. According to the Chrome team responsible for Web Vitals, what counts is the score at the 75th percentile of all page views in the field data. Google Search Console drops FID on March 12. PageSpeed Insights and CrUX keep it for a six-month transition period.

Who should care about this the most? WooCommerce stores, especially in the run-up to Black Friday, and sites built on heavy themes. They are full of interactive elements: the cart, product filters, variant selection, live search. Each of them runs JavaScript after a click. And that is exactly the moment INP measures.

How to check INP on a WordPress site step by step

Measuring takes a few minutes. Free Google tools are enough, plus data from your monitoring provider if needed. Go step by step:

1. Enter your home page URL in PageSpeed Insights, then your most important pages: categories, product pages, the cart.
2. Read the section with real user data, which comes from CrUX.
3. Open the Core Web Vitals report in Google Search Console, where URLs are grouped by similar issues.
4. If you use a RUM tool (Real User Monitoring, i.e. measurement on visitors’ devices), ask the provider whether it already supports INP.

Check mobile and desktop separately. Phones have weaker processors, so they usually score worse (no surprise there). Oh, and with low traffic CrUX may have no data about your site at all. In that case your own RUM is the only source of real measurements.

How to read the score: good, needs improvement, poor

INP gets one of three ratings. The goal? A „good” rating for most visits. A good score means the page responds smoothly and nothing is on fire. Needs improvement signals that some interactions clearly lag, usually on specific page types. A poor rating means customers wait after clicking and they feel it. That’s where you start cleaning up.

Also check where the score comes from. The lab test in PageSpeed Insights simulates a single device and doesn’t perform typical interactions. It’s useful for diagnosis, not for assessment. What counts is the CrUX field data, because it shows how real users experience the site on different phones and computers.

What most often hurts responsiveness on a WordPress site?

In practice it’s almost always JavaScript. It keeps the browser busy right when the user wants to click something. The most common culprits:

• heavy plugin scripts loaded on every page, including where they aren’t needed,
• complex page builders with many layers of code,
• third-party scripts: chats, analytics, ad pixels,
• long JavaScript tasks blocking the browser’s main thread,
• a large DOM, meaning thousands of HTML elements to recalculate after every change.

How do you find the culprit? The simplest way is on a copy of the site: disable plugins one at a time and measure the effect. Boring, but it works. The Performance tab in Chrome DevTools will show you which script runs after a click. The Chrome team has also prepared separate materials on diagnosing and fixing INP issues.

How to improve INP in WordPress and WooCommerce

Trimming JavaScript gives the biggest gains. Fewer plugins, and the ones that stay load their scripts only where they are used. A contact form doesn’t need to load code on a product page. Neither does a reviews module on the „About us” page. I recommend delaying third-party scripts until the first interaction or loading them after the content is displayed. One more thing: a lighter theme plus regular plugin updates often solve the problem without further digging, because developers improve code performance too.

Does your store need a custom feature? In my view it’s better to order lightweight code right away than to add yet another generic plugin to the collection. K4.pl offers such solutions, together with licences that include updates and support. Measure the score again after every change. Just stay patient: field data is collected with a delay, so you’ll see the effect in CrUX and Search Console only after a few weeks.

Measure INP regularly in PageSpeed Insights and Search Console. Start with your highest-traffic pages, then the cart and product pages. This metric tells you something very concrete: whether a button responds right away or the customer has to click a second time. Good responsiveness means fewer abandoned purchases, not just a green dot in a report.

Frequently asked questions