Page experience and speed, in Google’s words

Frame 1 · Build decisionsSix decisions made at the drawing stage

Each decision below comes from web.dev or Google. Two terms recur. Largest Contentful Paint (LCP) reports when the largest image, text block or video visible in the viewport has rendered. Cumulative Layout Shift (CLS) measures the largest burst of unexpected layout shifts, content moving while the page is open.

• Give every image and video its size. web.dev says to always include width and height attributes, or to reserve the space with CSS aspect-ratio, so the browser can hold the right amount of room while the file loads.
• Never lazy-load the main image. web.dev’s warning is direct: lazy-loading the LCP image “will always lead to unnecessary resource load delay”.
• Put the main image in the HTML. The browser should be able to find the LCP resource in the page source. If it is only referenced from a CSS or JavaScript file, web.dev suggests preloading it with a high fetch priority.
• Reserve room for anything that arrives late. Ads, embeds and iframes without dimensions, and content injected after load, are among the most common causes of a poor CLS.
• Treat web fonts as part of the layout. A font that renders larger or smaller than its fallback can shift the page.
• Size images for the screen. Google notes that images are often the largest part of a page’s size, and points to image optimisation and responsive images as the way to keep pages sharp and fast. Sheet 05 covers images in full.

Frame 2 · Where the time goesThe four parts of a slow LCP

web.dev breaks the time to Largest Contentful Paint into four parts in sequence. Naming them shows where a build can help.

1. Part 1 · Time to First ByteFrom the visitor starting to load the page to the browser receiving the first byte of the HTML.
2. Part 2 · Resource load delayFrom that first byte to the browser starting to load the main image or other LCP resource.
3. Part 3 · Resource load durationThe time taken to load that resource itself.
4. Part 4 · Element render delayFrom the resource finishing loading to the element being fully rendered.

If the largest element is text in a system font, parts 2 and 3 are zero: there is nothing to fetch.

Frame 3 · The targetsThe three numbers to aim for

Google Search Central gives one target for each of the three vitals.

One point for whoever tests the build: lab tools such as Lighthouse load a page with no visitor, so they cannot measure INP, and web.dev uses Total Blocking Time as the lab stand-in. It adds that lab testing is the best way to try a change before release, but is “not a substitute for field measurement”.

Frame 4 · Beyond the vitalsThe rest of page experience

Google’s page experience guide asks site owners not to focus on only one or two aspects, and says that answering yes to these questions means “you’re probably on track”. In its own words:

• “Do your pages have good Core Web Vitals?”
• “Are your pages served in a secure fashion?”
• “Does your content display well on mobile devices?”
• “Does your content avoid using an excessive amount of ads that distract from or interfere with the main content?”
• “Do your pages avoid using intrusive interstitials?”
• “Is your page designed so visitors can easily distinguish the main content from other content on your page?”

Its answers to common questions keep the vitals in proportion:

• There is no single page experience signal; Google’s core ranking systems look at a variety of signals.
• Good results in the Core Web Vitals report or third-party tools do not guarantee a top ranking, and chasing a perfect score “just for SEO reasons may not be the best use of your time”.
• Other page experience aspects do not directly help a page rank higher, but they can make a site more satisfying to use, which Google says is generally aligned with what its ranking systems seek to reward.
• Google “always seeks to show the most relevant content, even if the page experience is sub-par”; where many helpful pages are available, a great page experience can contribute to success.

Speed, in Google’s account, is one part of page experience, and it is judged on the visitors who actually arrive.