PageSpeed scores and Core Web Vitals are both Google metrics for site speed. They are often aligned with each other, meaning that if you have a good PageSpeed score, you should also have good Core Web Vitals. But, since there are some fundamental differences, they can also tell different stories. It’s important to understand how each of them works and how to best consider them for your site.
Core Web Vitals is a set of specific metrics that Google thinks represents the speed and user experience of your site. This can affect your SEO and Google rankings.
PageSpeed scores do not have any direct impact on how Google perceives the speed of your site. The PageSpeed score is supposed to help you get an idea of your site’s performance and diagnose issues.
A lot of top ranking sites have less than perfect PageSpeed scores. Take Amazon for example. They pass all their Core Web Vitals, but the PageSpeed scores are very average – 62 on mobile, 76 for desktop.
So what’s going on here? How can the same site have diverging results on different tests?
The Difference: Simulation Vs. Real Life, Synthetic test vs field data
The most fundamental difference between PageSpeed scores and Core Web Vitals is this:
PageSpeed is a synthetic lab test whereas Core web vitals is based on field data that is, actual user experiences, over a rolling 28 day period.
A synthetic test means that it’s not based on a real visitor loading a page in their browser. It’s an automated test that uses a simulation of a user visit, with specific parameters like browser type, connection speed, device, location.
Field data is based on what real users actually experience when they visit your site in Chrome. Google tracks that info and it becomes the basis of the Core Web vitals scores. It takes 28 days for Core Web Vitals to update based
If most of your site visitors have similar conditions (device, connection speed) to those used in the PageSpeed test, there’s a stronger possibility your Core Web Vitals will correlate.
But since Core Web Vitals is based on a wide range of experiences, they can be influenced by outliers or different conditions (slow connections, uncached pages etc).
A critical distinction is that a synthetic test like PageSpeed cannot interact with your page so it won’t show if there is Cumulative Layout Shift (CLS) or other issues once you get scroll down the page. That kind of CLS can drag down your Core Web Vitals but wouldn’t show up in a PageSpeed test.
How to see your Core Web Vitals data
If your site gets enough traffic, you’ll see this information at the top of your PageSpeed report and in your Google Search Console account.
What to do if you don’t have Core Web Vitals data?
If your site doesn’t get enough traffic but you want to get an idea of how real visitors are experiencing the site then you can look into Real User Monitoring (RUM) tools. This will get you closer to the Core Web Vitals data that Google is using to assess your site.
Real user monitoring is based on actual visits from actual people loading your site in their browser. The data is generated by adding a script into your site from a RUM provider. This script can then extract much more information about user experience. So you will get data about the real life experiences of your visitors; for example, how long is the Largest Contentful Paint (LCP) really for most visitors? Is there hidden Cumulative Layout Shift (CLS) that PageSpeed can’t show? With this information you can proactively fix issues and improve performance.
Some examples of RUM services:
- Request Metrics (has a limited free plan)
- CoreDash
- Debug Bear
RUM services are generally paid, so if you don’t want to pay for the tracking, you could do some manual work instead.
Recreate different conditions in your browser
By using the developer tools in your browser you can try to recreate various scenarios, and browse your site to see how it performs.
For example, you can choose a mobile device (not just responsive view) and a slow internet connection and see if your site is still fast. Make sure to scroll the whole page to check the full user experience.
What is PageSpeed good for?
If the PageSpeed score doesn’t influence Google rankings, why are we all obsessed with it? Some people consider it to be only a vanity metric, but it does have uses.
Some data is better than no data
Owners of smaller sites that don’t have Core Web Vitals data need some way to assess the speed of their site, so a synthetic lab test like PageSpeed is helpful. It provides some specific recommendations that, when addressed, do indeed help the real speed of your site.
Controlled environments are useful
When you use a tool like PageSpeed you are testing in the same controlled environment every time. So you are getting an “apples to apples” comparison. That means it’s good for checking to see what changes when you make adjustments to your page. There can be small fluctuations in PageSpeed results for various reasons but generally speaking, you can test a page before and after making changes to it and get a good idea if your change was positive or negative for performance.
Fast feedback
Since Core web vitals takes 28 days to update, using PageSpeed or other synthetic tools is a good way to get faster feedback on the effects of changes.
Synthetic lab tests and real user data each have their place in your performance testing. The key is just to understand what each one is doing and what kind of data you’re getting.