Performance
Core Web Vitals and site speed: what to fix first
3 August 2026 · 8 min read · Coselli, Bournemouth
Three metrics, a handful of fixes, and a measurable jump in conversions. What Core Web Vitals actually measure and how to improve them without a rebuild.
Speed is a small ranking factor and an enormous conversion factor. Improving it is one of the few changes that reliably increases revenue whether or not rankings move.
The three metrics
| Metric | Measures | Good |
|---|---|---|
| LCP — Largest Contentful Paint | When the main content appears | Under 2.5s |
| INP — Interaction to Next Paint | How fast the page responds to input | Under 200ms |
| CLS — Cumulative Layout Shift | How much the layout jumps around | Under 0.1 |
Google grades these on real visitor data over a 28-day window, not on lab tests. A perfect Lighthouse score with poor field data means your test conditions are nicer than your customers' phones.
Fixing LCP
- 01Identify the LCP element — usually the hero image or headline.
- 02If it is an image: serve modern formats, size it correctly for the viewport, and preload it. Never lazy-load the LCP image.
- 03Serve HTML from the server so text paints before JavaScript arrives.
- 04Self-host fonts, subset them, and use font-display: swap.
- 05Cut render-blocking CSS and defer non-critical scripts.
Fixing INP
INP is dominated by long JavaScript tasks blocking the main thread. The usual culprits are analytics bundles, chat widgets, cookie banners and heavy animation libraries loaded eagerly.
- Audit third-party scripts and remove any whose data nobody reads.
- Load chat and marketing widgets on interaction or after idle.
- Code-split so a page ships only the JavaScript it needs.
Fixing CLS
- Set explicit width and height (or aspect-ratio) on every image, video and embed.
- Reserve space for banners, ads and consent notices rather than injecting them above content.
- Preload web fonts so the fallback swap does not reflow the page.
Images: still the biggest single win
On most small business sites, images account for the majority of page weight. Resize to the largest size actually displayed, export as WebP or AVIF, keep hero images under 200KB and everything else well under 100KB, and lazy-load anything below the fold.
Measuring properly
- 01Use the Core Web Vitals report in Search Console for field data — that is what counts.
- 02Use PageSpeed Insights on individual templates, not just the homepage.
- 03Throttle to slow 4G and mid-tier mobile in DevTools when testing locally.
- 04Re-test after every deployment that touches templates or third-party scripts.
Set a budget and defend it: hero under 200KB, total JavaScript under 200KB compressed, LCP under two seconds on 4G. Budgets stop the slow creep that makes fast sites slow again within a year.
Frequently asked
- Do Core Web Vitals actually affect rankings?
- They are a real but modest signal, most influential as a tie-breaker between comparable pages. The bigger, more reliable payoff is conversion rate.
- My Lighthouse score is 95 but Search Console says poor. Why?
- Lighthouse is a lab test on a simulated device; Search Console reports real visitors on real phones and networks. Always trust field data.