How to tell if your website is slow on a phone

The quickest way to tell is to open your site on your phone with Wi-Fi turned off and watch how long the main part of the page takes to show up. Then run your address through Google’s free PageSpeed Insights test and look at the mobile results. Google’s line is 2.5 seconds. If the biggest thing on the screen takes longer than that to appear, the page needs work.

Test it yourself in two minutes

First, the simple test. Step away from your office Wi-Fi so you’re on cell data. Open your home page and count the seconds. Then tap your phone number and your menu. Did anything lag? Did the page jump while you were trying to tap?

Then, the measured test.

1. Go to pagespeed.web.dev.
2. Type in your web address and press Analyze.
3. Read the Mobile results.

You may see two kinds of results. One is what real visitors experienced over the last 28 days. Google only shows it when it has enough data, so a small local site may not have any. The other is a lab test. According to Google’s notes on the tool, it loads your page the way a mid-range phone on a mobile network would. That’s a tougher test than your office computer on Wi-Fi.

What the numbers mean

The Performance number is a score from 0 to 100. Google counts 90 or above as good, 50 to 89 as needing improvement, and below 50 as poor.

Google also tracks three measurements it calls Core Web Vitals:

• Largest Contentful Paint is how long the biggest photo or block of text takes to show up. Good is 2.5 seconds or less.
• Interaction to Next Paint is how fast the page reacts when you tap. Good is 200 milliseconds or less.
• Cumulative Layout Shift is how much the page jumps around while it loads. Good is 0.1 or less.

The lab test only loads the page. Nobody taps anything. So the tap-speed number shows up only when Google has real-visitor data for your site.

Two more things to keep in mind. The score moves a little from one run to the next, so test more than once. And you don’t need a perfect 100. Google’s own documentation calls a perfect score “extremely challenging to achieve and not expected.”

Why it matters

A phone has less power than a desktop computer, and a cell connection can be slow. Google’s guide for web developers says sites that load quickly keep visitors better than sites that don’t.

It matters to Google too. Google says it uses the mobile version of a site for indexing and ranking. It also says Core Web Vitals are used by its ranking systems.

I want to be careful here, because speed gets oversold. That same page says good scores don’t guarantee a top ranking, and that Google “always seeks to show the most relevant content, even if the page experience is sub-par.” Speed won’t rescue a page that doesn’t answer the customer’s question. It’s one part of a good site, not a magic switch.

What usually causes it

Google’s guides point to the same few problems.

• A big photo at the top. If the biggest thing on your screen is a photo, a heavy file means a long wait. Google’s guide to speeding up that first big element says to compress images and use modern formats like AVIF or WebP.
• Add-ons from other companies. Google’s list of third-party scripts includes social sharing buttons, video embeds, ads and analytics. It calls them a common cause of slowdowns.
• A slow server. Before anything shows up, your phone has to hear back from the computer that hosts your site. If that takes a while, everything after it is late too.
• Things that load late and shove the page around. Google says the most common causes of a jumpy page are images, ads and embeds with no size set, plus web fonts.

What fast looks like

Here’s a real number to compare against. On October 12, 2026, I ran Lighthouse, the test behind PageSpeed Insights, on the website for Phlix Photos, an app I make. On the mobile test it scored 100 for performance. The biggest thing on the page showed up in 867 milliseconds, and nothing jumped around.

That’s one simple page on one day. But it shows what’s possible when a site is built light.

A quick checklist

• Open your site on cell data and count the seconds
• Run it through PageSpeed Insights and read the Mobile results
• Write down the performance score and the Core Web Vitals
• Look at the big photo at the top of your home page
• Count the add-ons: social buttons, video embeds, ads and tracking scripts

Don’t want to read the report yourself? My free Site Check runs the same Google test and gives you the top five fixes in plain English.