Reading a PageSpeed report without the jargon

Run your store through Google PageSpeed Insights and you get a big number, four coloured circles and a long list of “opportunities” written for developers. Here’s how to read it as a store owner: what matters, what doesn’t, and what to hand to whoever maintains your theme.

Two reports in one

PageSpeed Insights actually shows two different things, and mixing them up causes most of the confusion:

• Field data, at the top, comes from real Chrome users who visited your page over the past 28 days. It only appears when Google has enough visits. This is what Google uses when it assesses page experience.
• Lab data, below, is a single simulated test run on the spot. On mobile it emulates a mid-range phone on a slow connection. It’s great for diagnosing problems, but it’s one run, and it varies.

If the two disagree, trust the field data for “how are we doing” and the lab data for “why”.

The three numbers that matter

Core Web Vitals are Google’s chosen measures of real user experience:

• LCP (Largest Contentful Paint): how long until the main content, often your hero image or product photo, appears. Good is 2.5 seconds or less.
• INP (Interaction to Next Paint): how quickly the page responds when someone taps or clicks, like choosing a size. Good is 200 milliseconds or less. It only comes from field data.
• CLS (Cumulative Layout Shift): how much the page jumps around while loading. Good is 0.1 or less.

Each is judged at the 75th percentile of visits, so most of your visitors, not just the ones on fast Wi-Fi, need a good experience.

What the big score means

The 0 to 100 Performance score is a weighted summary of the lab test only. It’s useful for tracking changes over time on the same page, but it isn’t a ranking factor in itself, and it’ll bounce a few points between runs. Don’t chase 100; chase green Core Web Vitals.

The other three circles (Accessibility, Best practices and SEO) are automated checklists. They’re worth fixing, but a perfect SEO score there only means the basics are in place.

Common Shopify culprits

• Oversized hero images. A 2 to 3 MB banner is the most common cause of slow LCP. Upload images at the size they’re displayed, and let Shopify serve modern formats.
• Too many apps. Each app can add scripts to every page, even pages where it does nothing. Remove apps you don’t use; uninstalling doesn’t always remove leftover theme code, so check.
• Sliders and pop-ups. Auto-playing carousels and pop-ups that load late are common causes of poor INP and CLS.
• Web fonts. Several font families and weights add up. Two weights of one family usually suffice.

A simple routine

1. Test four page types: home, a product, a collection and a blog post.
2. Save the results as your baseline.
3. Re-test after every theme change or new app install.
4. Hand the specific findings, not the score, to whoever maintains your theme.