How WebP Improves Core Web Vitals
Core Web Vitals are no longer just a buzzword you hear in SEO conversations. They are one of the most visible ways Google measures whether a page actually feels fast and stable to real people. At the same time, images are still the heaviest assets on most websites, especially blogs, portfolios and product galleries.
That is where WebP quietly becomes a powerful ally. It is not just another file format – it is a very practical way to improve metrics like Largest Contentful Paint (LCP), Cumulative Layout Shift (CLS) and First Input Delay (FID). In this article we will keep things simple and look at how WebP helps each of these vitals in day‑to‑day projects.
1. A Quick Reminder: What Are Core Web Vitals?
Google currently focuses on three main metrics under the Core Web Vitals label:
- LCP (Largest Contentful Paint) – when the main content becomes visible.
- FID (First Input Delay) – how quickly the page responds when a user first interacts.
- CLS (Cumulative Layout Shift) – how stable the layout is while the page loads.
In many layouts, a large hero image, banner or product shot is the element that defines LCP. If that image is heavy or blocked by slow loading, the page will feel sluggish even if your HTML and CSS are perfect.
2. LCP: WebP Makes the “Big Image” Cheaper
Because WebP compresses more efficiently than JPEG and PNG, the browser has less work to do when fetching and decoding the largest image on the page. When you replace a 500 kB hero JPEG with a 200 kB WebP version, you are cutting the payload that directly influences LCP.
On a typical connection this can shave several hundred milliseconds off the time it takes for the main content to appear. The best part is that the change is invisible to visitors – the picture simply appears faster and with the same perceived quality.
3. FID: Freeing the Main Thread
First Input Delay is mostly about JavaScript, but image formats do play a role. Very large or unoptimized images can keep the main thread busy longer than necessary, especially on low‑end mobile devices. WebP reduces decoding cost, which means the browser has more breathing room to react quickly when the user taps or scrolls.
If you combine WebP with a lighter JavaScript bundle and defer non‑critical scripts, FID issues often disappear without heavy refactoring.
4. CLS: Preventing Layout Jumps
WebP does not automatically fix layout shifts, but it makes it easier to design a stable page. Because files are smaller, you can comfortably set explicit width and height attributes for your images, knowing that they will still load quickly. With defined dimensions, the browser can reserve the right amount of space before the image is fetched, so the content around it no longer jumps.
5. Putting It into Practice
A realistic workflow looks like this:
- Export or resize your images to a sensible maximum width.
- Convert them to WebP using a tool such as ImagePulser or ImageDocker.
- Add
widthandheightattributes in your HTML to lock the layout. - Use
loading="lazy"for images below the fold.
None of these steps require a new framework or a redesign, but together they move your LCP, FID and CLS in the right direction. For many sites this is enough to go from “needs improvement” to “good” in PageSpeed Insights.
6. Measuring the Difference
Before and after switching to WebP, it is worth running a few measurements. PageSpeed Insights and your browser’s performance panel can show you the exact timing of LCP and how much the layout shifts over time. Even on a modest connection, the difference between uncompressed JPEG and well‑optimized WebP is very noticeable.
Conclusion
WebP is not a magic switch that fixes every performance problem, but it is a very effective part of a modern Core Web Vitals strategy. By lowering the cost of your heaviest elements – images – it makes every other optimization more impactful. If you have not tried it yet, start with a single important page, convert its assets to WebP, and watch how much smoother it feels to use.
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