Mobile performance explained

A large share of your visitors are on their phones, and Google indexes and ranks your site from its mobile version. And yet mobile performance is often worse than desktop, because phones have slower processors, less memory, and often a patchy connection. This guide explains what you need to know.

Why mobile is different from desktop

When you test your site on a desktop computer, you have:

A fast processor
Plenty of memory
A wired or strong Wi-Fi connection
A recent browser

When a visitor uses a phone, they might have:

A budget Android phone from three years ago
3G or a weak 4G signal
A browser that has to work harder to run JavaScript

The same page can feel dramatically slower on a mid-range phone with an average connection than on a powerful laptop. This is why Google's own testing simulates a mid-tier phone on a mobile network for its mobile results, and an emulated desktop on a wired connection for desktop.

Google's mobile-first indexing

Google "crawls" (scans) websites to decide how to rank them. In Google's words: "Google uses the mobile version of a site's content, crawled with the smartphone agent, for indexing and ranking. This is called mobile-first indexing."

This means:

If your mobile site is slow or broken, your Google visibility suffers, even for desktop searches
Speed problems that only appear on mobile are worth treating as ranking problems
Content that is missing from the mobile version may not count for SEO. Google is explicit that if the mobile version has less content than the desktop one, you can expect to lose some traffic. Tucking content into accordions or tabs is fine; leaving it out is not.

See Mobile-friendliness & SEO for more on the SEO side.

What makes mobile performance worse

Slower processors

Running JavaScript is demanding work for a browser, and Google names it as often the primary driver of how interactive a page feels. The same work takes noticeably longer on a budget phone than on a laptop, so sites with lots of JavaScript (page builders, interactive features, third-party scripts) take the biggest hit on mobile.

Slower connections

4G and even 5G connections are variable. In a city center, your visitor might have excellent speeds. In a suburb, in a building, or in a rural area, they might be on 3G or a congested network. Each extra kilobyte of page weight is more expensive on a slow connection.

Smaller CPU and memory

Phones have less RAM than computers. A page that opens 20 browser tabs' worth of scripts may run fine on a laptop but cause a phone to stutter or crash.

How PageSpeed Insights tests mobile performance

For its mobile results, PageSpeed Insights runs Lighthouse against a simulated mid-tier device on a mobile network, rather than against your own phone on your own Wi-Fi. This is intentionally conservative. It reflects the experience of visitors on average hardware, not top-of-the-line phones.

This is why your PageSpeed score may look worse than what you experience on your own phone. Your phone is probably faster than average. The test also runs from a Google data center in North America, Europe or Asia, which is another reason two runs can differ.

Test on your phone too

Use your site on a real mid-range Android phone (or ask us to test it) to get a feel for how your visitors experience it. Emulated tests are useful but real devices reveal issues that simulators miss.

What helps mobile performance

Many of the same fixes that help desktop speed help mobile even more:

Optimize imagesSmaller images are especially impactful on slow connections
Minimize JavaScriptEvery heavy plugin hits mobile harder
Use a CDNReduces distance, which matters more on high-latency mobile connections
Fast hostingA slow TTFB is painful on mobile
Avoid autoplay videoVideo is very expensive on mobile connections

Additionally, your site should be responsive: Google's own definition is a site that serves the same HTML at the same URL whatever the device, but displays it differently depending on the screen size. Done properly, that includes handing phones smaller images rather than the same large ones as desktop. Most well-built modern sites are already responsive.

Common questions

My site looks fine on my iPhone. Why does it score poorly?

iPhones are premium devices with fast processors. A budget Android phone will give a very different experience. PageSpeed Insights deliberately simulates a mid-tier device instead. Also, you probably have your site cached in your browser, so repeat visits are much faster.

Does this mean I need a separate mobile website?

Almost certainly not. Google supports three ways of building for mobile (responsive design, dynamic serving, and separate mobile URLs) but it recommends responsive design, because it is the easiest pattern to implement and maintain. One responsive site that adapts to the screen size is the right answer for nearly every small business. Most modern website platforms produce responsive sites out of the box; if you are unsure what yours does, ask us.

Should I hide content on mobile to make it faster?

Hiding an element with CSS usually does not stop the browser downloading it, so it rarely makes a page faster. If you want to reduce page weight on mobile, serve smaller images and lighter assets rather than just hiding things. And be careful: content missing from the mobile version can cost you search traffic, because that is the version Google indexes. Talk to us if you want to investigate mobile-specific optimizations.