LCP, INP, CLS — you've probably seen these acronyms in a PageSpeed Insights report and scrolled past them. Here's what they actually mean for a SaaS marketing site, which ones you should care about, and what moves the needle.
Why this matters beyond SEO
Core Web Vitals became a Google ranking signal in 2021. But the more practical reason to care is that these metrics correlate with what users actually experience — and therefore with conversion rates.
The three metrics and what they actually measure
LCP — Largest Contentful Paint
LCP measures how long it takes for the largest visible element on the page to render. On most SaaS marketing sites, this is the hero image or the hero section background.
Good: under 2.5 seconds on mobile. Poor: over 4 seconds.
What moves LCP on Next.js sites:
- Use
next/imagewithpriorityon the hero image. This preloads the image as a high-priority resource. - Serve images in WebP or AVIF format (Next.js does this automatically with
next/image). - Use a CDN for images — Vercel's image optimisation pipeline handles this.
- Avoid CSS background images for LCP candidates.
INP — Interaction to Next Paint
INP measures the latency from a user interaction (click, keypress, tap) to the next visual response.
Good: under 200ms. Poor: over 500ms.
For a largely static marketing site, INP is usually not a problem. Where it surfaces: heavy third-party scripts that block the main thread.
What moves INP: Audit third-party scripts. Defer non-critical scripts with strategy="lazyOnload" in Next.js's Script component.
CLS — Cumulative Layout Shift
CLS measures how much the page layout shifts as it loads.
Good: under 0.1. Poor: over 0.25.
What moves CLS: Add explicit width and height attributes to all images. With next/image, this is handled automatically — you must provide width and height props.
A realistic priority order for most SaaS sites
- Fix LCP on the homepage and top landing pages (hero image optimisation, CDN delivery)
- Fix any obvious CLS issues (add dimensions to images, check for layout-shifting embeds)
- Audit third-party scripts and defer anything non-critical
- Verify everything in Search Console field data, not just lab tests
