Lab data and field data
Performance numbers come from two kinds of measurement. Lab data comes from a test: a computer loads the page once under fixed conditions and measures it. Field data comes from real visits: Chrome measures how the page behaved for actual people on their own phones and computers.
Both are useful, and they rarely show the same number. A page can look slow in the lab and fine for real users, or the other way around.
Why it matters
Section titled “Why it matters”The lab test is always available and gives the same conditions every time, so it’s good for spotting problems and checking whether a change helped. But it’s one load, on one simulated device, by a visitor who never clicks or scrolls.
Field data shows what visitors actually experienced, including returning visitors, fast and slow phones, and everything that happens after the page has loaded. That’s what Google uses to rate a page’s Core Web Vitals. It needs enough Chrome visitors, though, and it looks back over 28 days, so it reacts slowly to changes.
Why the numbers differ
Section titled “Why the numbers differ”Browser: Real visitors use all kinds of devices and screens. On a large screen, a different element may be the largest one, which changes Largest Contentful Paint. Visitors also interact: they click, type and scroll. That’s why Interaction to Next Paint only exists in field data, and why layout shifts further down the page, for example from images without a fixed size, often show up only there.
Cache: The lab test always starts with an empty browser cache. Returning visitors often have the theme’s stylesheets, fonts and scripts stored already, so their pages load faster. On the server side, logged-in WordPress users and visitors with items in a shopping cart usually skip the page cache, so they wait longer than the test does.
Network: The lab simulates one fixed connection from a Google data center. Real visitors connect from different places, on fast fiber or patchy mobile networks, and some of them are far from your server.
WordPress: Personalized content, cookie notices and ads often look different for each visitor, or load later than in the lab. What the test sees is one version of the page.
In Permo
Section titled “In Permo”On its PageSpeed page, Permo shows the lab data per device (Mobile and Desktop) at the top and the field data underneath, in the Real users section. The real-user values come from the Chrome UX Report: for each metric, the value that 75% of Chrome visits reached or beat over the last 28 days. Google only provides them when enough Chrome users have visited, so many smaller sites see lab data only.