Why Is My Mobile PageSpeed Score Lower Than Desktop?

It’s one of the first things people notice: desktop says 92, mobile says 58. Same website, same page, same moment. Is something broken?

Usually not. The two tests are measuring very different situations, on purpose.

How does PageSpeed Insights test mobile vs desktop?

PageSpeed Insights runs Google’s Lighthouse tool twice, with different settings:

	Mobile test	Desktop test
Device	Emulated mid-range phone with small screen	Desktop-size screen
Processor	Deliberately slowed down to mimic a typical phone	Not slowed down
Network	Throttled to simulate a slower mobile connection	Fast, wired-style connection
Result	The same page takes much longer	More forgiving conditions

So the mobile test is essentially asking: “What happens when an ordinary person on an ordinary phone, on mobile data, opens this page?” That’s a much harder exam.

Why the slowed-down processor matters so much

Most people assume the slower network is the main reason. It’s a big part, but the processor is often the bigger one.

Modern websites run a lot of JavaScript: page builders, sliders, tracking tags, chat widgets. A powerful laptop chews through it in moments. A mid-range phone takes several times longer. That’s why Total Blocking Time and responsiveness often look fine on desktop and poor on mobile.

Heavy images hurt too. A 2 MB hero image downloads quickly on fibre. On a throttled mobile connection, it can add seconds to your Largest Contentful Paint.

How big a gap is normal?

There’s no official rule, but in our experience:

• A gap of 10 to 30 points is very common, even on well-built sites.
• A gap of 40+ points usually means the page leans heavily on JavaScript or large images.
• A small gap with both scores high is a sign of a genuinely lean site. It’s achievable: our own homepage scores 100 on both.

Which score should you care about?

Mobile. Almost always.

First, Google uses mobile-first indexing, which means it primarily uses the mobile version of your site for indexing and ranking.

Second, that’s where your customers are. Across South Africa, a large share of people browse mainly on their phones, often on prepaid data and patchy signal. Think of someone in a taxi on the N2 searching for a nearby panel beater. They’re not on fibre.

Desktop still matters for some audiences, particularly B2B buyers researching at work. But if you can only improve one, improve mobile. Desktop usually improves along the way.

How to close the gap

Shrink images for mobile. Serve smaller image sizes to smaller screens and use modern formats like WebP or AVIF. See our image optimisation guide.

Cut JavaScript. Remove unused plugins, delay non-essential scripts, and question every third-party widget.

Simplify the mobile layout. Keep everything important, but drop decorative extras: autoplay videos, big sliders, animation-heavy sections.

Improve hosting and caching. Server delays hurt both scores, but they compound on mobile.

Test on a real phone. Turn off Wi-Fi and load your site on mobile data. If it feels slow to you, it feels slow to customers.

Common misunderstandings

“Google is penalising my mobile site.” The score itself isn’t a penalty; it’s a diagnostic. For rankings, what counts is real-user Core Web Vitals and, above all, content relevance.

“My phone loads it fine.” You probably have a newer phone, good signal, and the site cached from previous visits. Your customers may not.

“A responsive theme means mobile is handled.” Responsive means the layout adapts to the screen. It doesn’t mean the page is light enough for a phone.

For a walkthrough of every number in the report, see how to read a PageSpeed Insights report.

When to get help

If your mobile score stays in the red after compressing images and removing plugins, the problem is often structural: a heavy theme or page builder. That’s when a performance-focused rebuild becomes the better investment, because every marketing channel you pay for sends people to that mobile experience.

Frequently asked questions

Why is my mobile PageSpeed score so low?

The mobile test simulates a mid-range phone with a slower processor on a throttled connection, so heavy images and JavaScript hurt much more than on desktop.

Is a mobile score of 70 good?

It’s in the “needs improvement” range, but a reasonable result for many business sites. Aim for 90+, and treat anything under 50 as urgent.

Does Google rank sites based on mobile or desktop?

Google uses mobile-first indexing, so the mobile version of your site is primarily used for indexing and ranking.

Can a site score 100 on mobile?

Yes. It takes a lean build, optimised images and minimal JavaScript, but it’s achievable.