Site Speed Optimisation and Core Web Vitals — What They Are and How to Improve Them

Page speed has been a confirmed Google ranking factor since 2010. In 2021 Google formalised its approach to page experience with the introduction of Core Web Vitals — a set of specific, measurable signals that assess how pages load, how stable they are, and how quickly they respond to user interaction. Understanding these signals and what drives them is an important part of technical SEO for any website competing in search.

For a broader overview of technical SEO topics, see the technical SEO guide.

What are Core Web Vitals?

Core Web Vitals are a set of three metrics Google uses to measure page experience. They are part of Google’s broader page experience signals and are included as ranking factors, particularly in competitive SERPs where other signals are closely matched.

The three metrics are:

Largest Contentful Paint (LCP)

LCP measures how long it takes for the largest visible element on a page — typically a hero image, a large heading, or a block of text — to finish rendering. It is a proxy for perceived load speed: how quickly does the page feel loaded to the user.

Google’s threshold for a good LCP score is under 2.5 seconds. Between 2.5 and 4 seconds is needs improvement. Above 4 seconds is poor. LCP is the Core Web Vital most commonly failed and often has the most direct impact on ranking.

Cumulative Layout Shift (CLS)

CLS measures visual stability — how much the page layout shifts unexpectedly during loading. When elements move around as the page loads, it creates a poor user experience and can cause accidental taps on mobile. A good CLS score is below 0.1. The most common causes are images without explicit dimensions, dynamically injected content, and web fonts that cause text to reflow on load.

Interaction to Next Paint (INP)

INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. It measures the time between a user interaction — a click, a tap, or a keypress — and the next visual response from the page. A good INP score is below 200 milliseconds. Poor INP is most commonly caused by heavy JavaScript execution that blocks the browser’s main thread.

Why site speed matters beyond Core Web Vitals

Core Web Vitals are the formal ranking framework, but page speed affects organic performance in other ways too. Slow pages increase bounce rates, which reduces engagement signals. They affect crawl efficiency — Googlebot has a crawl budget per site and may crawl fewer pages on a slow site. And for e-commerce and lead generation sites, page speed has a direct and well-documented impact on conversion rates independent of any SEO effect.

A one-second improvement in load time has been shown to significantly improve conversion rates on mobile, where connection speeds are more variable and user tolerance for slow pages is lower. Speed improvements therefore deliver both SEO and commercial value simultaneously.

Common causes of slow page speed

The most frequent causes of poor Core Web Vitals scores and slow page load times are:

  • Unoptimised images — oversized files, incorrect formats, or missing compression. Images are the single most common cause of poor LCP scores. Serving images in WebP or AVIF format with appropriate dimensions and compression is one of the highest-impact fixes available
  • Render-blocking resources — JavaScript and CSS files that prevent the browser from rendering the page until they have fully loaded. Deferring non-critical JavaScript and inlining critical CSS eliminates this bottleneck
  • Third-party scripts — analytics platforms, chat widgets, A/B testing tools, advertising pixels, and social media embeds each add load time and can contribute to both LCP delays and CLS. Auditing and removing unnecessary third-party scripts is frequently one of the quickest wins
  • Slow server response time (TTFB) — Time to First Byte measures how quickly the server responds to a request. A TTFB consistently above 600 milliseconds indicates a server, hosting, or caching issue that no amount of front-end optimisation will fully compensate for
  • Missing image dimensions — images without explicit width and height attributes cause layout shifts as the browser does not know how much space to reserve before the image loads, resulting in CLS failures
  • Excessive JavaScript execution — large JavaScript bundles, unused JavaScript, or long tasks on the main thread delay both rendering and interactivity, affecting LCP and INP
  • No caching — pages that are not cached at the server or CDN level force the server to generate a fresh response for every request, increasing TTFB and overall load time

How to diagnose site speed issues

Google provides two primary tools for diagnosing speed and Core Web Vitals problems:

Google Search Console’s Core Web Vitals report provides a site-wide view of which pages are failing LCP, CLS, or INP thresholds based on real user data (field data). This is the most reliable signal of how pages are actually performing for users and is the data Google uses for ranking purposes.

PageSpeed Insights gives page-level diagnostics combining real user data with a lab-based audit. It highlights specific opportunities — such as image compression savings, render-blocking resources, and unused JavaScript — with estimated impact. It is the most practical tool for identifying what to fix on individual pages.

For a site-wide view of which pages are underperforming and why, a technical SEO audit will include a full Core Web Vitals assessment alongside other technical findings.

Fixing site speed problems

The correct fix depends on the diagnosis. Image optimisation — compressing, resizing, and converting to modern formats — is almost always worth doing and rarely requires development input. Deferring render-blocking JavaScript and setting explicit image dimensions are similarly straightforward. Addressing TTFB, improving caching configuration, or reducing JavaScript bundle sizes typically requires closer collaboration with a development team.

It is worth noting that PageSpeed Insights scores are not the same as Core Web Vitals scores. The lab-based score in PageSpeed Insights is a useful diagnostic tool, but the field data from real users is what Google uses for ranking. A site can have a lower lab score but strong field data if its real-world users are on fast connections — and vice versa. Focus on the field data in Search Console as the primary signal.

For support with diagnosing and fixing speed issues as part of a broader technical review, find out more about my technical SEO services.

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