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LCP: Largest Contentful Paint Under 2.5 Seconds
LCP measures how long it takes for the largest visible element — almost always a hero image on affiliate posts — to load. Google's "good" threshold is under 2.5 seconds. Pages above 4 seconds are rated "poor" and earn a negative Page Experience signal. The fastest LCP fix is three changes applied simultaneously: convert the hero image to WebP format, add fetchpriority="high" to its img tag, and add a preload link in the document head.
Preload syntax: <link rel="preload" as="image" href="/hero.webp" fetchpriority="high">. This tells the browser to download the LCP image immediately, before parsing the rest of the HTML. Without preload, the browser discovers the image only after parsing the full HTML and CSS — often 800–1,200 milliseconds later. These three changes combined typically reduce LCP by 0.5–1.5 seconds on image-heavy affiliate posts.
INP: Interaction to Next Paint Under 200 Milliseconds
INP replaced FID as Google's interactivity metric in March 2024. INP measures the time from any user interaction — click, tap, keypress — to the next visual update. The "good" threshold is under 200 milliseconds. Affiliate sites typically fail INP because of heavy JavaScript from comment plugins, live chat widgets, social sharing buttons, and advertising scripts that run on the main thread and block user interactions.
Audit your JavaScript with Chrome DevTools Coverage tab. Open DevTools, go to Coverage, reload the page, and look for scripts where 70% or more of the code is unused on load. Adding defer or async attributes to non-critical scripts prevents them from blocking the main thread during page load. Replacing jQuery-dependent plugins with lightweight vanilla JavaScript equivalents typically cuts INP by 50–70% on plugin-heavy WordPress sites.
CLS: Cumulative Layout Shift Under 0.1
CLS measures unexpected visual shifts as the page loads — elements jumping position as fonts render, images load, or ads inject into the layout. The "good" threshold is under 0.1. The primary CLS causes on affiliate sites are images without explicit width and height attributes, late-loading ad slots, and web font swaps that change text dimensions before the custom font loads.
Fix images by always declaring width and height in HTML: <img src="product.webp" width="800" height="600" alt="...">. The browser reserves the exact space before the image loads, preventing layout shifts. Fix web fonts with font-display: swap in your @font-face declarations and add a font preload link to the document head. Fix ad slots by reserving fixed-height containers with CSS min-height before the ad JavaScript fills them.
Measuring Core Web Vitals with Free Tools
Google PageSpeed Insights at pagespeed.web.dev tests any URL and returns both lab data and field data for all three Core Web Vitals. Use PSI for initial diagnosis and for verifying fixes. The Opportunities and Diagnostics sections provide specific, prioritised recommendations ranked by estimated impact — work top-to-bottom through the list.
Google Search Console's Core Web Vitals report shows field data aggregated across all your pages, grouped by "Good", "Needs Improvement", and "Poor". It identifies which specific URLs fail and which issue type causes the failure. Fix the "Poor" pages first — they receive the strongest negative ranking signal. The Web Vitals Chrome extension shows real-time CWV scores on any page you visit, making it practical for comparing your pages against competitors'. Combine CWV fixes with a strong content foundation using insights from our free SEO tools workflow.
Image Optimisation: The Single Highest-Impact CWV Fix
Images cause LCP failures, CLS failures (when dimensions are missing), and slow overall page weight — making image optimisation the highest-ROI Core Web Vitals investment. Convert all images to WebP using Squoosh, ImageOptim, or your hosting platform's automatic conversion. WebP averages 25–35% smaller file sizes than JPEG at equivalent visual quality.
Add srcset attributes to serve appropriately sized images to different screen widths. A 1,200px-wide product image sent to a 375px mobile screen wastes three times the bandwidth needed. Use the sizes attribute to tell the browser the rendered display size, and provide WebP variants at 400w, 800w, and 1200w breakpoints. This single change typically reduces page weight by 40–60% and improves LCP by 0.3–0.8 seconds on mobile.
Server Response Time and Font Loading
Time to First Byte — how quickly your server responds to a request — affects LCP directly. A TTFB above 600 milliseconds makes it nearly impossible to achieve LCP under 2.5 seconds regardless of other optimisations. Use a CDN like Cloudflare to serve cached HTML from edge locations close to each visitor. Enable automatic static file caching and set long cache TTLs for images and fonts.
Google Fonts loaded via the standard stylesheet link block rendering until the CSS parses and the font files download. Replace the standard link tag with a preconnect to fonts.gstatic.com and load the stylesheet non-blocking using the onload pattern. This technique eliminates the font-related render-blocking delay on first load and typically improves LCP by 200–400 milliseconds. See our guide on ranking without backlinks for how technical performance ties into topical authority building.