Tested 2026-07-27 22:02:01 using Firefox 153.0 (runtime settings)
Use --filmstrip.showAll to show all filmstrips.
Click a frame to seek the video to that moment. The dot marks frames where the page changed visually.
The whole recording in one image: every pixel is tinted by how often it changed. Amber changed once, deep red changed ten or more times. The scale is fixed, so heatmaps from different runs compare directly.
Flat amber is a page that painted once and stayed put. Red speckle in the shape of the text means the same pixels repainted over and over, usually invisible rendering noise that inflates Last Visual Change and Speed Index. A solid red block is a genuinely restless area, like an ad or a carousel.
Rendering was not meaningfully delayed. First paint landed at 785 ms on a 444 ms first byte.
The coach helps you find performance problems on your web page using web performance best practice rules. And gives you advice on privacy and best practices. Tested using Coach-core version 9.2.1.
decodingAsyncThe page has 1 image (out of 1) without a decoding hint. Add decoding="async" to non-critical images so the browser can decode them off the main thread.
Setting decoding="async" on an <img> tells the browser it can decode the image off the main thread, which keeps the page responsive to user interactions while images are being processed. The default ("auto") leaves the choice to the browser. https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#decoding
modernImageFormatsThe page ships 1 image (out of 1) in JPEG/PNG/GIF without a modern alternative. Wrap them in a <picture> with a <source type="image/avif"> or "image/webp" before the legacy <img>, or serve modern formats from your image pipeline directly. AVIF and WebP usually deliver 25–50% smaller files at the same quality.
AVIF and WebP routinely deliver 25–50% smaller files than JPEG and PNG at the same perceived quality, and every browser version still under support understands at least one of them. Ship modern formats either through a <picture> element with <source type="image/avif"> / "image/webp" entries in front of the legacy <img>, or directly from a content-negotiating image pipeline that returns AVIF / WebP when the client accepts it. https://web.dev/articles/serve-images-webp
lcpImageHintsThe LCP image is missing fetchpriority="high". Adding it tells the browser to fetch the image with high priority instead of the default heuristic (which often deprioritises hero images that are loaded after the HTML has been parsed).
When the Largest Contentful Paint element is an image, the browser priority hints applied to that element directly affect the LCP metric. The image must NOT be loading="lazy" (that defers the fetch until near-viewport, which is the opposite of what an LCP image needs) and SHOULD be fetchpriority="high" (so the browser fetches it with high priority instead of guessing). https://web.dev/articles/fetch-priority
cacheHeadersLongThe page has 4 requests that have a shorter cache time than one year (but still a cache time).
Setting a cache header is good. Setting a long cache header (a year) is even better because the asset will stay in the browser cache across visits. For content-hashed URLs (e.g. app.4af2.css) you can safely use Cache-Control: max-age=31536000, immutable. For unversioned URLs that may change, use a revalidating strategy instead.
cumulativeLayoutShiftLayout Shift is not supported in this browser
Cumulative Layout Shift measures the sum total of all individual layout shift scores for unexpected layout shift that occur. The metric is measuring visual stability by quantify how often users experience unexpected layout shifts. It is one of Google Web Vitals.
viewportThe viewport meta tag does not contain width=device-width, the browser may use a desktop-width fallback.
The viewport meta tag tells the browser how to lay out the page on small screens. Without it (or without width=device-width) the page is rendered at a desktop fallback width and scaled down, which makes text unreadable on mobile. Disabling zoom (user-scalable=no, maximum-scale<=1) is also an accessibility regression. https://developer.mozilla.org/en-US/docs/Web/HTML/Viewport_meta_tag
unnecessaryHeadersThere are 6 responses that sets both a max-age and expires header. There are 7 responses that sets a server header.
Do not send headers that you don't need. We look for p3p, cache-control and max-age, pragma, server and x-frame-options headers. Have a look at Andrew Betts - Headers for Hackers talk as a guide https://www.youtube.com/watch?v=k92ZbrY815c or read https://www.fastly.com/blog/headers-we-dont-want.
referrerPolicyNo <meta name="referrer"> tag was found on the page. Set a Referrer-Policy response header (preferred) or add a meta tag, for example <meta name="referrer" content="strict-origin-when-cross-origin">.
Without an explicit referrer policy the browser falls back to the user-agent default and may leak the full URL of the previous page (including query strings) to every cross-origin request. Set a Referrer-Policy response header (preferred) or a <meta name="referrer"> tag in the document. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy
contentSecurityPolicyHeaderSet a Content-Security-Policy header to mitigate cross-site scripting attacks. You can start with a Content-Security-Policy-Report-Only header, which only reports violations rather than blocking them.
A Content-Security-Policy response header tells the browser which sources of script, style, and other content are allowed. The most effective form is a strict CSP using nonces or hashes together with strict-dynamic; the worst is a missing header, with unsafe-inline and unsafe-eval close behind. https://web.dev/articles/strict-csp
crossOriginEmbedderPolicyHeaderSet a Cross-Origin-Embedder-Policy header (typically require-corp or credentialless) on the document response to control cross-origin embedding.
Cross-Origin-Embedder-Policy (COEP) makes the page refuse to load cross-origin subresources unless they explicitly opt in via CORP or CORS. Together with Cross-Origin-Opener-Policy it puts the page in a cross-origin isolated context, which mitigates cross-window side-channel attacks (Spectre) and unlocks high-resolution timers and SharedArrayBuffer. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Embedder-Policy
crossOriginOpenerPolicyHeaderSet a Cross-Origin-Opener-Policy header (typically same-origin) on the document response to isolate the page from cross-origin windows.
Cross-Origin-Opener-Policy (COOP) lets a page sever its window-group ties to cross-origin documents that opened it or that it opens. Together with Cross-Origin-Embedder-Policy it puts the page in a cross-origin isolated context, which mitigates cross-window side-channel attacks (Spectre) and unlocks high-resolution timers and SharedArrayBuffer. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy
crossOriginResourcePolicyHeaderSet a Cross-Origin-Resource-Policy header (same-origin, same-site or cross-origin) on the document response to limit who may embed it.
Cross-Origin-Resource-Policy (CORP) is a per-response opt-in that tells the browser which origins are allowed to embed the resource. It blocks cross-origin or cross-site no-cors embedding (img, script, iframe, etc.) and is one of the building blocks of cross-origin isolation. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Resource-Policy
permissionsPolicyHeaderSet a Permissions-Policy header to control which browser features the page can use.
The Permissions-Policy response header (the successor to Feature-Policy) lets a site explicitly opt in or out of powerful browser features such as camera, microphone, geolocation, payment and clipboard. Setting a strict policy reduces the attack surface and limits what embedded third parties can do. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Permissions-Policy
referrerPolicyHeaderSet a referrer-policy header to make sure you do not leak user information.
Referrer Policy is a new header that allows a site to control how much information the browser includes with navigations away from a document and should be set by all sites. https://scotthelme.co.uk/a-new-security-header-referrer-policy/.
xContentTypeOptionsHeaderSet X-Content-Type-Options: nosniff on the document response to prevent MIME-sniffing.
X-Content-Type-Options: nosniff prevents browsers from interpreting files as a different MIME type than what is declared in the Content-Type header. This blocks a class of cross-site scripting and content-type confusion attacks and should be set on every response. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options
A snapshot of what the browser actually built for this page: the document, how big and deep the DOM tree is and what it kept in storage. Big, deep trees are slower to style and lay out, so the counts Chrome warns about carry a flag.
Document
What the page says it is and how large it rendered.
DOM structure
How much markup the browser has to build, style and lay out.
What the page stored on the client, and the network it was tested on.
Data collected using
Coach-core version 9.2.1. With updated code from
Webappanalyzer 2026-05-04. Use
--browsertime.firefox.includeResponseBodies html or
--browsertime.chrome.includeResponseBodies html to help Wappalyzer find more information about technologies used.
0.0s0.8s0.9s1.0s2.0sWhen the page main content is rendered, collected via the Largest Contentful Paint API. Read more about Largest Contentful Paint.
body#www-wikipedia-org > main > div:eq(0) > img
The LCP element is highlighted in the screenshot. If nothing is highlighted the element was removed before the screenshot or the LCP API couldn't find it.
The Largest Contentful Paint API matched this image:
Timing data the server chose to expose through Server-Timing response headers on the main document — typically backend time, cache status or experiment flags.
| Name | Duration | Description |
|---|---|---|
cache | 0 ms | hit-front |
host | 0 ms | cp3070 |
co_id | 0 ms | 2929263558 |
There are no custom configured scripts.
There are no custom extra metrics from scripting.
How the page is built.
| Header | Value |
|---|---|
date | Mon, 27 Jul 2026 09:27:36 GMT |
cache-control | s-maxage=86400, must-revalidate, max-age=3600 |
server | ATS/9.2.13 |
etag | W/"1d629-65738a87fd940" |
last-modified | Wed, 22 Jul 2026 20:05:17 GMT |
content-type | text/html |
content-encoding | gzip |
age | 45266 |
accept-ranges | bytes |
x-cache | cp3070 miss, cp3070 hit/3023694 |
x-cache-status | hit-front |
strict-transport-security | max-age=106384710; includeSubDomains; preload |
report-to | { "group": "wm_nel", "max_age": 604800, "endpoints": [{ "url": "https://intake-logging.wikimedia.org/v1/events?stream=w3c.reportingapi.network_error&schema_uri=/w3c/reportingapi/network_error/1.0.0" }] } |
nel | { "report_to": "wm_nel", "max_age": 604800, "failure_fraction": 0.05, "success_fraction": 0.0} |
x-client-ip | 138.201.135.103 |
content-length | 30635 |
x-request-id | 3c8dd77a-81ae-4e9b-b7f5-9d7b0dcba2f1 |
x-analytics | |
server-timing | cache;desc="hit-front", host;desc="cp3070",co_id;desc="2929263558" |
x-firefox-spdy | h2 |
| URL | Type | Transfer Size | Content Size |
|---|---|---|---|
| https://www.wikipedia.org/ | html | 31.4 KB | 0 b |
| https://www.wikipedi...rite-e49fbf32.svg | svg | 19.0 KB | 0 b |
| https://www.wikipedi...pedia-logo-v2.png | image | 16.6 KB | 0 b |
| https://www.wikipedi...dex-34f340e24a.js | javascript | 9.6 KB | 0 b |
| https://www.wikipedi...uch/wikipedia.png | image | 2.2 KB | 0 b |
| https://www.wikipedi...con/wikipedia.ico | favicon | 2.0 KB | 0 b |
| https://www.wikipedi...ie9-507b16b6be.js | javascript | 1.7 KB | 0 b |
Transfer size82.7 KB
Content size0 b
Requests7
| Content | Header Size | Transfer Size | Content Size | Requests |
|---|---|---|---|---|
| html | 1.5 KB | 31.4 KB | 0 b | 1 |
| javascript | 2.4 KB | 11.3 KB | 0 b | 2 |
| image | 2.1 KB | 18.9 KB | 0 b | 2 |
| favicon | 1.0 KB | 2.0 KB | 0 b | 1 |
| svg | 1.2 KB | 19.0 KB | 0 b | 1 |
| Total | 8.1 KB | 82.7 KB | 0 b | 7 |
| Domain | Total download time | Transfer Size | Content Size | Requests |
|---|---|---|---|---|
| www.wikipedia.org | 1.623 s | 82.7 KB | 0 b | 7 |
| type | min | median | max |
|---|---|---|---|
| Expires | 1 hour | 1 day | 1 year |
| Last modified | 4 days | 4 days | 3 weeks |
Want to dig deeper? Download the Firefox Gecko profile and drop it into profiler.firefox.com.