Core Web Vitals for Marketing Sites: What Actually Affects Conversion (Not Just SEO)

Core Web Vitals get talked about almost entirely as an SEO checkbox—a score to satisfy, not a thing that affects whether visitors actually convert. That framing undersells them. These metrics correlate directly with conversion rate on marketing sites, independent of anything to do with search rankings.

The Three Metrics, in Plain English

LCP (Largest Contentful Paint)

How long it takes the biggest visible element, usually a hero image or headline, to appear. If your hero section is slow to render, visitors are staring at a blank or half-loaded page during the exact moment you're trying to make a first impression.

INP (Interaction to Next Paint)

How responsive the page feels when someone actually clicks something. A slow INP means a visitor clicks a CTA and the page hesitates before responding, which reads as broken, even if it technically isn't.

CLS (Cumulative Layout Shift)

How much the page jumps around as it loads. A classic example: a visitor goes to tap a CTA, and an ad or image loads in above it, shifting the button down—so they tap the wrong thing. This isn't just annoying, it's a direct conversion leak.

Why They Matter More Than Your SEO Score

Search ranking is a slow-moving, indirect incentive to fix these. Conversion rate is immediate and direct: a visitor who experiences a slow, jumpy page forms an impression of the brand before reading a single word of copy. Fixing Core Web Vitals is fixing first impressions, not just a technical score.

The Fixes That Move the Needle Most on Marketing Sites

• Image optimisation. Hero images are usually the single biggest LCP cost on a marketing site—compress and size them correctly before anything else.
• Font loading strategy. Custom fonts that block rendering until they load are a common, invisible cause of a slow LCP and layout shift.
• A third-party script audit. Chat widgets, analytics tags, and marketing pixels each add weight and can block interactivity—most sites are running scripts nobody's audited in over a year.
• Careful use of hero video and animation. Autoplay video and heavy animation libraries look impressive but are frequently the biggest single cause of a poor INP score on otherwise well-built sites.

How to Check Your Own Site

Run your key pages—homepage, your main landing pages—through Google's PageSpeed Insights, using the mobile score specifically, since that's where most traffic and most of the problems live. A score consistently below 70 on mobile is usually costing you conversions, not just ranking positions.

FAQ

Do Core Web Vitals actually affect conversion rate, or just SEO?

Both. Search ranking is the indirect effect; a visitor's first impression of a slow or jumpy page is the direct one, and it happens on every visit regardless of how they arrived.

What's the fastest fix for a slow marketing site?

Almost always image optimisation—hero and above-the-fold images are the most common single cause of a poor score, and usually the easiest to fix without a full rebuild.

Does using a page builder or CMS make Core Web Vitals worse?

Often, yes—many page builders load more script and style weight than a custom-built page needs. It's not disqualifying, but it's worth auditing specifically if your site is built on one.

If your site's performance might be costing you conversions, see our web design and development work, or book a free audit and we'll show you exactly what's slowing you down.