Lighthouse 13.6 Rolls Into PageSpeed Insights: Scores Likely to Rise

PageSpeed Insights now runs on Lighthouse 13.6.0, released Oct. 27, and the change alters how the tool simulates page loading — meaning many sites will see higher performance scores without touching a line of code.

Chrome's Web Performance Developer Advocate, Barry Pollard, confirmed the rollout on Bluesky. He said the update improves the simulated FCP (First Contentful Paint) and simulated LCP (Largest Contentful Paint), both of which "will be more accurate than before and generally result in higher scores."

A page may receive a different PageSpeed Insights score with nothing on it changing, because the test itself changed. Pollard added that sites heavily dependent on JavaScript are more likely to feel the impact.

Which sites see the biggest shifts?

Pollard's comments respond to a developer who reported that single-page apps (SPAs) had seen a big improvement in results since the update, citing react.dev as an example. Pollard opened his post with "True!" and added:

"Note this is not SPA-specific (and unrelated to other SPA things we've been working on), but more likely to impact JS-heavy sites."

He also drew a line between this update and Chrome's experimental soft navigation metrics, covered in October. Those metrics measure real user experience with data from actual Chrome users via the Chrome User Experience Report (CrUX). Lighthouse, by contrast, runs a simulated test.

What changed in the simulation?

By default, Lighthouse loads a page at full speed and then estimates how it would load on a slower connection. Lighthouse's throttling documentation says this simulated throttling matches the PageSpeed Insights setup.

The 13.6.0 release notes list several changes to Lantern, the simulation engine behind those estimates, all aimed at improving accuracy:

• Render-blocking detection. Lighthouse 13.5 used each request's fetch priority to decide whether it blocked rendering. In 13.6, Lantern uses Chrome's own render-blocking label for each request, falling back to priority only when that label is missing.
• Text-based LCP. When the LCP element is text appearing in the same frame as the page's first content, Lantern now estimates that LCP from the same requests and page tasks it uses for FCP — instead of from a set that could include every request made before LCP.

Developer Ivan Akulov demonstrated the old simulation's flaw in a GitHub issue. His test page, built with Framer, had only text above the fold, so one element counted as both FCP and LCP. Lighthouse reported a much later LCP than FCP for that text. Worse, preloading the page's JavaScript modules — which improved interactive time for the real page — made the simulated LCP worse.

Pollard thanked Akulov in a reply, saying he "kicked off some of these improvements." The Lighthouse team closed the issue as completed in the Lantern upgrade listed in the 13.6 release notes.

What about the ARD audit?

Lighthouse 13.6 also updates the Agentic Resource Discovery (ARD) audit introduced in 13.5 and covered in September. The 13.5 release looked for an ARD manifest under the old ai-catalog names even though the spec had moved to ard.json — so sites using only the new name without an Agentmap line in robots.txt showed the audit as Not Applicable.

Lighthouse 13.6 now checks, in order:

• an Agentmap line in robots.txt,
• a rel="ard" link tag or Link header,
• /.well-known/ard.json,

and only then falls back to the ai-catalog names. If it finds the manifest only at an ai-catalog location, it notes that ARD consumers are not required to check there.

Lab scores vs. field data

The performance score PageSpeed Insights shows comes from Lighthouse-generated lab data — a simulated load on a single device under fixed network conditions. Google's documentation warns that "having good lab data does not necessarily mean real-user experiences will also be good."

Field data, shown in the same report, reflects real visits over the last 28 days as reported by CrUX. Every report lists the Lighthouse version used, next to the emulated device.

Why the version switch matters

Scores from before and after the Lighthouse 13.6 update weren't measured with the same simulation. Part of a higher lab score on a freshly optimized page may come from the new version itself, and lab-score charts tracked over time may show a jump exactly at the switch to 13.6.

Before reporting an improvement, check that both reports name the same Lighthouse version, and compare the field data.

What to watch next

Lighthouse's release notes say the team expects 13.6 to ship in the DevTools of Chrome 159, currently in Chrome's desktop Dev channel and Canary. As of Nov. 2, Chrome's release schedule lists Chrome 159 for stable release on Dec. 8. An open Lighthouse GitHub issue is also investigating cases where Lantern's simulated LCP runs far above measured LCP, using large JavaScript and SPA sites as examples — worth monitoring if your lab and field numbers still disagree after the update.