Core Web Vitals

Core Web Vitals measure what a visitor feels while a page loads and responds. Google publishes three:

• Largest Contentful Paint (LCP): how long until the biggest image or text block in view has rendered. Good is 2.5 seconds or less.
• Interaction to Next Paint (INP): how quickly the page responds to clicks, taps and key presses across the whole visit. Good is 200 milliseconds or less. INP replaced First Input Delay as a Core Web Vital in March 2024.
• Cumulative Layout Shift (CLS): how much content jumps around unexpectedly. Good is 0.1 or less.

Each is judged at the 75th percentile of real page loads, split by mobile and desktop. That data comes from the Chrome UX Report and shows up in Search Console and PageSpeed Insights. A Lighthouse score in your own browser is a lab test, useful for debugging but not the number Google uses.

Why it matters for a SaaS website

Most SaaS sites lose their scores in the same few places: a hero video or a huge product screenshot slows LCP, chat widgets, tag managers and A/B testing scripts block the main thread and hurt INP, and cookie banners, late-loading fonts and embeds push content down and add layout shift. These are also the pages paid traffic lands on, so a slow page costs money twice: in ad spend that bounces and in the page experience signal for organic search.

How DesignMe handles it

We set a performance budget in design, not after launch. Heroes get a poster image and video loads only when it’s on screen, images ship in modern formats at the right sizes, fonts are preloaded with fallbacks sized to match, and every third-party script has to earn its place. On builds we check field data in Search Console after launch, not only Lighthouse before it. When a site needs to be as fast as possible and edited by AI agents, we build it in Astro, which ships no JavaScript by default.

Example

Say a pricing page loads in 1.9 seconds on desktop but scores poorly on mobile. Field data shows LCP at 4.1 seconds, caused by a 900 KB hero illustration, and CLS at 0.24 from a cookie banner that pushes the page down. Serving the illustration as a sized AVIF with a preload, and moving the banner to an overlay, brings both into the good range within one 28-day data window.

Common mistakes

• Chasing a 100 Lighthouse score. Fix what real users experience in field data first.
• Testing on a fast laptop. Most problems show on mid-range phones over mobile networks.
• Letting scripts pile up. Each marketing tool adds main-thread work. Audit the tag manager every quarter.
• Forgetting templates. One slow blog or integration template can drag down hundreds of URLs at once.