Website Speed and Revenue: What the Research Says and How to Fix a Slow Site

By Krzysztof Szymaniak · November 4, 2026 · 6 min read

Most small business owners know a slow website is annoying. Fewer know how directly it connects to revenue, or which numbers to look at to fix it. In this article I’ll walk through what the research actually says about speed and conversions, how Google measures page experience today with Core Web Vitals, and a practical order of fixes for a typical WordPress or WooCommerce site.

What the research says about speed and sales

It’s easy to find made-up statistics about page speed online, so I’ll stick to two sources you can read yourself.

Google and Deloitte: “Milliseconds Make Millions”

In 2020 Google commissioned Deloitte to study how small changes in mobile site speed affect user behavior. The study covered 37 brands across Europe and the US and about 30 million user sessions. In the published report, a 0.1-second improvement in mobile site speed was associated with an 8.4% increase in conversions for retail sites and a 10.1% increase for travel sites. Retail consumers also spent almost 10% more.

A tenth of a second is not something a visitor consciously notices. That’s the point: speed works below the level of conscious decisions, by removing tiny moments of friction across the whole visit.

Vodafone: a controlled A/B test

Correlation studies are useful, but an A/B test is stronger evidence. Google’s web.dev team published a case study from Vodafone, which ran an A/B test focused on Web Vitals. A 31% improvement in Largest Contentful Paint (LCP) led to 8% more sales, a 15% improvement in lead-to-visit rate and an 11% improvement in cart-to-visit rate.

Your site isn’t Vodafone and your results will differ. But both sources point in the same direction: faster pages tend to convert better, and the effect is measurable.

How Google measures speed today: Core Web Vitals

“Speed” used to mean one number, usually load time. Today Google uses a set of metrics called Core Web Vitals that try to capture what users actually experience. There are three.

Largest Contentful Paint (LCP)

LCP measures loading: how long it takes for the largest visible element, often a hero image or headline, to appear. A good LCP is 2.5 seconds or less.

Interaction to Next Paint (INP)

INP measures responsiveness: how quickly the page reacts visually when someone taps a button, opens a menu or types into a form. A good INP is 200 milliseconds or less. INP replaced First Input Delay as a Core Web Vital in March 2024, and Google explains the metric in detail in its INP guide.

Cumulative Layout Shift (CLS)

CLS measures visual stability: how much content jumps around while the page loads. A good CLS score is 0.1 or less. If you’ve ever tried to tap a link and hit an ad that suddenly appeared, you’ve experienced bad CLS.

Google assesses each metric at the 75th percentile of real visits, separately for mobile and desktop. In other words, your site needs to be fast for most visitors, not just on your office Wi-Fi.

Does speed affect Google rankings?

Yes, but it’s not a magic lever. Google’s Search Central documentation on Core Web Vitals says they are used by its ranking systems and recommends that site owners achieve good scores. Its page experience guidance also makes clear that relevant, helpful content still matters most. A fast page with weak content won’t outrank a slower page that answers the question much better.

The practical takeaway: treat speed as a conversion and user-experience project first. Any ranking benefit is a bonus. If you also want to fix crawling, indexing and structured data, that’s the job of technical SEO.

How to check your own site in five minutes

Open PageSpeed Insights and paste in your homepage URL.
Look at the top section first: “Discover what your real users are experiencing.” That’s field data from real Chrome users, if your site has enough traffic.
Note LCP, INP and CLS for mobile. Mobile is usually where the problems are.
Repeat for your most important page, such as a product page, a service page or your contact form page.
Scroll down to the diagnostics. They’ll hint at the cause: large images, render-blocking scripts, too much JavaScript.

If the field data section is empty, your site doesn’t have enough traffic for Chrome’s dataset. In that case use the lab score as a rough guide, and remember it’s a simulation.

The usual suspects on WordPress and WooCommerce sites

When I audit slow WordPress sites, the causes repeat. Here they are, roughly in order of how often I see them.

Oversized images

A 4,000-pixel photo straight from a camera, displayed at 800 pixels wide. Converting images to modern formats like WebP or AVIF, serving the right size and lazy-loading images below the fold often fixes LCP on its own. One exception: don’t lazy-load the hero image, because that delays LCP.

Too many plugins loading scripts everywhere

A slider plugin loads its JavaScript on every page, even where there’s no slider. Multiply by twenty plugins and the browser is busy parsing code nobody uses. This is the most common cause of poor INP.

Heavy page builders and themes

Some builders wrap every element in several layers of markup and ship large CSS and JavaScript bundles. You can tune them, but sometimes a lean theme with native blocks is the better long-term answer.

Cheap or overloaded hosting

If your server takes over a second to respond, nothing on the front end can fully compensate. Check Time to First Byte in the PageSpeed diagnostics.

Third-party scripts

Chat widgets, heatmaps, multiple analytics tags and ad pixels each cost a little. Together they can cost a lot. Audit what you actually use. Load tracking only after cookie consent, which is good for both privacy and speed.

Layout shifts from fonts and embeds

Web fonts that swap late, images without width and height attributes, and cookie banners that push content down all hurt CLS. They’re usually quick to fix.

A practical order of fixes

Measure the pages that make money, not just the homepage.
Fix images and caching first. They’re low risk and often the biggest win.
Remove or replace plugins you don’t need, and stop the rest from loading scripts on pages that don’t use them.
Defer non-essential third-party scripts and gate tracking behind consent.
Look at hosting if server response time is still slow.
Consider a rebuild if the theme or builder is the bottleneck. Patching an old stack forever can cost more than a website redesign on a lean foundation.

Then keep it fast. Sites slow down over time as plugins, scripts and content pile up. A website care plan with regular checks stops that drift.

Is it worth paying for speed work?

That depends on your traffic, your conversion rate and what a customer is worth to you. I built a free website ROI calculator so you can check with your own numbers. Enter your monthly visits, current conversion rate, a conversion rate you think is realistic after improvements, and your average customer value. It shows how a change in conversion rate translates into revenue. It doesn’t predict anything. It just does the math transparently.

If you run an online store, speed work on product and checkout pages usually has the most direct impact. That’s a big part of my WooCommerce development work.

Summary

Google and Deloitte linked a 0.1s mobile speed improvement with 8.4% more retail conversions. Vodafone saw 8% more sales after improving LCP by 31% in an A/B test.
Core Web Vitals targets: LCP 2.5s or less, INP 200ms or less, CLS 0.1 or less, measured at the 75th percentile of real visits.
Speed is a ranking signal, but content relevance still matters more.
On WordPress, start with images, caching and plugin cleanup before bigger changes.

Want a second pair of eyes? Request a free website audit. I’ll check speed, mobile usability, security and SEO, and send you a short, prioritized list of fixes within 2 business days. You can also read more about my website speed optimization service.