Core Web Vitals in Practice: What Actually Moved the Needle

Core Web Vitals in Practice: What Actually Moved the Needle

We took a client site from an eleven-second load to 1.1 seconds. Here is the ordered list of changes and roughly what each one contributed.

Performance advice is usually a long undifferentiated checklist. In practice a small number of changes deliver most of the improvement. Here is what we did on a recent rebuild, in order of impact.

Images - roughly 60 percent of the total gain

The original site served full-resolution JPEGs, some over 3 MB, scaled down in the browser. We converted to AVIF with WebP fallbacks, generated responsive sizes, added correct width and height attributes to eliminate layout shift, and lazy-loaded everything below the fold. Page weight fell by 84 percent. This one category of work delivered more than everything else combined.

Render-blocking resources - about 20 percent

Six stylesheets and eleven synchronous scripts sat in the head, including two chat widgets and three analytics tags. We inlined critical CSS, deferred the rest, delayed third-party scripts until after first interaction, and removed two tags nobody could identify an owner for.

Fonts - about 10 percent

Four families in nine weights, self-hosted without preloading. We cut to two families in four weights, preloaded the critical ones, subsetted to Latin, and used font-display: swap. Largest Contentful Paint improved by nearly a second on its own.

Server and caching - about 10 percent

Time to First Byte was 900 milliseconds because every page ran uncached database queries. Object caching in Redis, full-page caching for anonymous visitors, and a CDN in front brought TTFB under 200 milliseconds.

What barely mattered

Minifying already-gzipped CSS saved a few kilobytes. Removing unused CSS helped less than expected. Neither was worth the effort compared to the image work.

Measure on the right device

Test on a mid-range Android phone on a throttled connection, not on your own laptop over office fibre. Lab tools are useful for diagnosis, but field data from real users is what Google actually scores you on - and the difference between the two is frequently large.

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