Core Web Vitals made simple!
We get it, Core Web Vitals have weird names and acronyms. They’re confusing.
Plus, there are many additional metrics to measure website speed and usability, outside of Core Web Vitals.
CLICK TO ENLARGE: Core Web Vitals made simple.
And to add to that, Core Web Vitals changed again in 2024. Here, we simplify everything you need to know in 2025 about these important metrics, and we show you how to check your own site.
Measuring your auto parts site’s speed
There are many metrics to measure the speed and usability of an auto parts and accessories website (or even a dealership site to sell cars). Core Web Vitals includes three metrics that measure site speed. These three are the most important to start with in this article. We cover some additional metrics farther down, scroll down to see more.
Largest Contentful Paint (LCP)
Translation: The user sees something useful as a website starts to load.
Largest Contentful Paint (LCP) is simply the amount of time it takes for the largest piece of content on a webpage to become visible to the user.
This content could be an image, a video, or a block of text. LCP is important for user experience because it gives an idea of when the most significant part of the page is loaded and ready for the user to see and interact with. A faster LCP generally means a quicker and more enjoyable experience for website visitors. That’s why Google considers this to be important.
Cumulative Layout Shift (CLS)
Translation: Things move on the page as other things load.
Cumulative Layout Shift (CLS) is simply how much things on a page move around as the page loads.
It quantifies the visual stability of a webpage. When you’re trying to read or interact with a page, it’s frustrating if buttons or text suddenly shift, leading to unintended clicks or confusion. That’s why Google considers this to be important. Usually CLS makes things on a page shift down, but things can shift left or right, too.
CLS helps assess and improve the user experience by measuring how stable the page layout as it loads. A lower CLS score generally means a more visually stable and user-friendly website.
Interaction to Next Paint (INP)
Translation: The website responds to clicks quickly.
Interaction to Next Paint (INP) measures how quickly a website responds to a mouse click, pressing a key or a tap on a touchscreen. INP replaced First Input Delay in March 2024.
Interaction to Next Paint is a continuous measurement, while First Input Delay just measures the first interaction. Google likes INP because it measures how a user continues to interact with a site, not just the first interaction.
Important non-Core Web Vitals metrics made simple
At the beginning of this article we said there were many additional metrics to measure a website that are not part of Core Web Vitals. They’re still important to know.
Time to First Byte (TTFB)
Translation: server speed.
Time to First Byte is a measurement of how quickly a server responds to a new web page visitor and starts to load a page. That’s a bit of an oversimplification but to keep things simple, we’ll go with server response time. Note that it’s possible to get an artificially slower TTFB in a test. That’s because tools like PageSpeed Insights and GTmetrix add their own latency to simulate the real world (scroll down and read about lab data vs. field data).
First Paint (FP)
Translation: I think I see something happening.
The First Paint metric measures the time it takes for you to notice something happening when you visit a web page.
First Contentful Paint (FCP)
Translation: I’m now sure something is happening.
First Contentful Paint is the first meaningful thing rendered on the page. It reassures the user that something is happening. This happens before Largest Contentful Paint.
Time to Interactive (TTI)
Translation: I can interact with this page and click on things.
Time to Interactive measures how quickly a user can click on buttons, links, forms, see the navigation menu or scroll.
DOM Content Loaded (DCL)
Translation: The page is done loading…for now…
DCL means the HTML part of your web page finishing loading and all JavaScript scripts have fired, including any scripts that may have been deferred. It doesn’t wait for images, asynchronous scripts, most css files or iframes.
Deprecated Core Web Vitals
First Input Delay (FID)
Translation: Users think they can interact, but can’t yet.
Interaction to Next Paint replaced First Input Delay in March 2024 and FID went away in September 2024. First Input Delay (FID) was a measurement on how long it took for a user to be able to interact with a website for the first time.
It looked at the delay between a user’s first interaction, such as clicking a button or a link, and the website’s response to that action. FID helped assess the responsiveness of a website but it was replaced with INP because FID was a measurement of the first interaction, while INP was a continuous measurement. INP is a better measurement of how a user is able to engage with a website for the duration of the time spent on the website.
Core Web Vitals checkers
There are several ways to measure the speed and usability/responsiveness of your website. There are specific metrics for site speed, they’re free to use and they’re all available four main ways through the Google ecosystem: Lighthouse Core Web Vitals, a tool that measures speed, SEO best practice and usability; Google Search Console, a tool that monitors your site’s performance, ranked keywords, organic clicks and impressions; Google’s PageSpeed Insights; and Google’s Chrome User Experience Report (CrUX for short), a tool that measures site speed and usability.
There are also a lot of non-Google tools to monitor Core Web Vitals including GTmetrix and WebPageTest.org.
The CrUX Dashboard is really handy because you can create your own report in Google LookerStudio.
Here are the differences with the four Google Core Web Vitals checkers (on a phone? scroll right):
| Metric | Lighthouse | Google Search Console | Chrome User Experience Report (CrUX) | PageSpeed Insights |
| First Paint |  | |||
| First Contentful Paint | | | | |
Largest Contentful Paint (LCP) | | | | |
| Cumulative Layout Shift (CLS) | | | | |
| Interaction to Next Paint (INP) | | | | |
| Time to First Byte (TTFB) | | | ||
| DOM Content Loaded | | |||
| Total Blocking Time (TBT) | | |||
| Speed Index | | |||
| HTTPS | |
Website usability metrics and Core Web Vitals in the past
CLICK TO ENLARGE: Important web usability metrics including Core Web Vitals, before Interaction to Next Paint (INP) replaces First Input Delay (FID).
This graphic shows the important metrics to measure site speed using the 2023 version of Core Web Vitals.
Google prefers the new Interaction to Next Paint (INP) metric, because it measures the responsiveness during a user’s entire time on a website. That’s why it replaced First Input Delay (FID) in March 2024, which only measures the first time a website is able to respond.
Why are these web performance KPIs important?
Don’t take our word for it that web performance KPIs (key performance indicators) are important. Check out WPOstats.com if you aren’t convinced that web page metrics and Core Web Vitals are important. It’s full of case studies with before/after results showing improvements in conversion rate vs. improvements in site speed and performance.
Improving site speed and performance usually leads to a lower bounce rate and a better conversion rate. That’s backed up by countless studies, including our own, comparing conversion rate to site speed.
Are your web performance KPIs real? Field vs. lab data
OK, we showed you how to measure your site’s performance, and we showed you the important metrics to know. But if you run a test, how do you know the results are valid?
Great question!
There are two data sources to be aware of: field data and lab data.
Field data comes from thousands of real site visits to your website, by real users, on all pages, using a rolling 28 day average (assuming your site has visitors and isn’t new). The term “real user monitoring” or RUM is sometimes used, it’s the same thing.
Field data is stored in a massive, massive public dataset, pulling from millions of websites.
Lab data uses one test, on one page, in a controlled environment. Some of the sources for metrics in the table above use field data, some use lab data.
Here’s a comparison of data (on a phone? scroll right):
| Lighthouse | Google Search Console | Chrome User Experience Report (CrUX) | PageSpeed Insights | WebPageTest | GTmetrics | |
| Data Source: | Lab data | Field data | Field data | Both | Lab data | Lab data |
Field data is great for reviewing the overall performance of your auto parts and accessories website over time. It’s a great high-level point of view and it can show you overall trends.
Lab data is great for debugging specific pages.
PageSpeed Insights field data vs. lab data
It’s important to know, if you use Google’s PageSpeed insights, it reports both lab and field data. This is one of the most misunderstood thing about PageSpeed Insights, which looks like it is reporting lab data. When you run a page through PageSpeed Insights, the performance scores you see from 0 to 100 are using lab data. The Core Web Vitals stats come from field data.
We hope this article was helpful. Now, go test your website!
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