The fast path (what to do first)
If you want improvements quickly, do this in order:
- Identify the LCP element (hero image, headline, or large block).
- Fix obvious CLS (images without dimensions, late UI inserts, font swaps).
- Reduce long tasks that hurt INP (heavy JS, big hydration, expensive handlers).
- Validate with field data after release.
Measure correctly: lab vs field
You need both:
- Lab data (Lighthouse, DevTools) helps you debug fast and compare changes.
- Field data (real users) is what search engines use for Core Web Vitals evaluation.
Minimum measurement workflow
- Run Lighthouse locally for a baseline.
- Use DevTools Performance panel to find long tasks and heavy scripts.
- Ship changes behind small iterations.
- Validate with real-user metrics (RUM) or Search Console.
LCP quick wins (Largest Contentful Paint)
LCP is often dominated by your hero content: large images, headlines, or above-the-fold blocks. Your goal: make the LCP element render earlier and faster.
1) Fix hero images first
- Serve the right size (avoid sending a 2000px image to a 390px phone).
- Use modern formats (WebP/AVIF when available).
- Preload the hero image if it’s the LCP element.
- Avoid lazy-loading the hero image (lazy-load below-the-fold).
2) Reduce render-blocking resources
- Keep critical CSS small and avoid heavy blocking styles.
- Defer non-critical scripts.
- Use font loading strategy (see CLS section too).
3) Cache and compress aggressively
- Enable server-side compression (Brotli/gzip).
- Use long-cache headers for static assets with fingerprinted filenames.
- Make sure TTFB is reasonable (slow backend can dominate LCP).
LCP mini checklist
- ✅ Hero image is optimized + correct size
- ✅ Hero image is not lazy-loaded
- ✅ Critical CSS is small
- ✅ JS is deferred where possible
- ✅ Static assets cached
CLS quick wins (Cumulative Layout Shift)
CLS happens when elements move unexpectedly. The fastest wins come from reserving space and avoiding late inserts.
1) Always reserve space for images and embeds
- Set width/height on images (or CSS aspect-ratio) so the browser can allocate space.
- Reserve space for iframes, ads, and embeds.
2) Stabilize font loading
- Use a sensible fallback stack.
- Prefer consistent metrics fonts or adjust fallback metrics if needed.
- Avoid “late” font swaps that cause reflow.
3) Don’t insert UI above existing content
- Cookie banners, promos, and toolbars should not push content down after render.
- If you must show a banner, reserve space from the start or overlay it.
CLS mini checklist
- ✅ Images have width/height (or aspect ratio)
- ✅ Ads/embeds reserve space
- ✅ Font loading is stable
- ✅ No late content inserted above the fold
INP quick wins (Interaction to Next Paint)
INP is about responsiveness. Users click, tap, type—and the page should respond quickly. The biggest enemy is long tasks on the main thread.
1) Cut long tasks
- Split big bundles (code-splitting).
- Defer non-critical JavaScript.
- Lazy-load expensive components (editors, charts, maps).
2) Keep event handlers light
- Don’t do heavy compute directly inside click/input handlers.
- Batch DOM updates and avoid forcing layout repeatedly.
- Move heavy work off the main thread when possible.
3) Reduce hydration pain
- Hydrate only what you need above the fold.
- Delay non-critical interactive widgets.
- Prefer server rendering for initial UI when it helps.
INP mini checklist
- ✅ No big long tasks during interaction
- ✅ Non-critical JS deferred
- ✅ Heavy widgets lazy-loaded
- ✅ Event handlers are light
Framework notes (React / Next.js)
- Keep the homepage light. Delay charts/editors/third-party widgets.
- Prefer server-rendered content for above-the-fold text so it appears quickly.
- Be strict with third-party scripts. Ads/analytics can hurt INP and LCP.
- Optimize images (responsive sizes, modern formats, correct priority).
Before-you-ship checklist
- ✅ Identify the LCP element and optimize it
- ✅ Remove CLS sources (images/fonts/late banners)
- ✅ Reduce JS and long tasks for better INP
- ✅ Cache + compress static assets
- ✅ Validate changes with lab tools, then confirm field data after release
FAQ
What should I fix first for Core Web Vitals?
Start with the LCP element (often the hero image), then eliminate obvious CLS, then reduce main-thread work to improve INP.
Is lab data or real-user data more important?
Use lab tools to debug quickly, but validate with field data because Core Web Vitals are based on real-user metrics.
What’s the fastest way to reduce CLS?
Reserve space for images/ads/embeds, stabilize font loading, and avoid inserting UI above existing content after render.
What’s the fastest way to improve INP?
Reduce long tasks: defer non-critical JS, split bundles, lazy-load heavy components, and keep event handlers light.