2006326 - 51.93 - 4.66% espn largestContentfulPaint / reddit ContentfulSpeedIndex + 1 more (Linux) regression on Wed December 10 2025

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[Treeherder Bug Filer]

Perfherder has detected a browsertime performance regression from push cfb49f1fc071252668878674a263ea356567a693. As author of one of the patches included in that push, we need your help to address this regression.

Please acknowledge, and begin investigating this alert within 3 business days, or the patch(es) may be backed out in accordance with our regression policy. Our guide to handling regression bugs has information about how you can proceed with this investigation.

If you have any questions or need any help with the investigation, please reach out to [e-mail]. Alternatively, you can find help on Slack by joining #perf-help, and on Matrix you can find help by joining #perftest.

Regressions:

Ratio	Test	Platform	Options	Absolute values (old vs new)	Performance Profiles
52%	espn largestContentfulPaint	linux1804-64-shippable-qr	fission warm webrender	259.55 -> 394.35	Before/After
8%	reddit largestContentfulPaint	linux1804-64-shippable-qr	fission warm webrender	154.43 -> 166.02	Before/After
6%	bing-search SpeedIndex	linux1804-64-shippable-qr	bytecode-cached fission warm webrender	296.03 -> 312.92	Before/After
5%	reddit ContentfulSpeedIndex	linux1804-64-shippable-qr	fission warm webrender	288.91 -> 302.38	Before/After

Improvements:

Ratio	Test	Platform	Options	Absolute values (old vs new)	Performance Profiles
22%	google-search LastVisualChange	linux1804-64-shippable-qr	cold fission webrender	855.42 -> 666.95	Before/After
18%	espn fcp	linux1804-64-shippable-qr	fission warm webrender	208.42 -> 170.53	Before/After
14%	espn FirstVisualChange	linux1804-64-shippable-qr	fission warm webrender	173.73 -> 148.61	Before/After
11%	espn PerceptualSpeedIndex	linux1804-64-shippable-qr	fission warm webrender	271.26 -> 241.62	Before/After
10%	imdb FirstVisualChange	android-hw-a55-14-0-aarch64-shippable	warm webrender	276.98 -> 249.36
...	...	...	...	...	...
3%	facebook largestContentfulPaint	linux1804-64-shippable-qr	fission warm webrender	279.73 -> 272.07	Before/After

Details of the alert can be found in the alert summary, including links to graphs and comparisons for each of the affected tests.

If you need the profiling jobs you can trigger them yourself from treeherder job view or ask [e-mail] to do that for you.

You can run all of these tests on try with ./mach try perf --alert 47836

The following documentation link provides more information about this command.

Comment 1

[BugBot [:suhaib / :marco/ :calixte]]

Set release status flags based on info from the regressing bug 2002960

Comment 2

[Tooru Fujisawa [:arai]]

The amount of regression on espn itself is not something worrisome.
For some reason, the website seems to have two modes,

• (a) The content is immediately shown from blank
• (b) The content shows the transition effect with fade-in from blank

In the (a) mode, the FCP and LCP are almost same. This has been happening mostly before the patch.
in the (b) mode, the FCP becomes earlier than (a), and of course LCP is very late, given the entire content is re-painted until the fade-in transition finishes.
So, the LCP regression and the FCP improvement are actually connected, and presumably the FCP happening earlier makes the transition visible.

Then, if I open the real website in my Nightly, I see the transition effect. So the (b) is more reflecting the actual user experience.
Anyway, I don't think this is a real regression.

Other regressions don't look like caused by the patch.