Why is my website slow on mobile? Causes, fixes and the Core Web Vitals
Why small-business sites are slow on phones (huge images, plugins, cheap hosting, heavy scripts), the Core Web Vitals targets, and the fixes in order.
Published: 6 min read Asen Panayotov, B63
Short answer: most small-business sites are slow on phones for the same few reasons: oversized photos, too many plugins and scripts, and cheap shared hosting. Google’s targets are the main content showing within 2.5 seconds, taps responding within 200 milliseconds and nothing jumping around as the page loads.
The frustrating part is that the site often feels fine on the office computer. Phones are slower, mobile connections are less reliable, and most small-business visitors arrive on a phone. This guide covers how to test it properly, what usually causes it, and which fixes pay off first.
How to test your site’s speed properly
- Run a phone test, not a desktop one. Our free website check runs Google’s PageSpeed Insights test on a simulated mid-range phone and explains the results in plain language. It takes about 30 seconds and only asks for your email. Google’s PageSpeed Insights shows the same test in technical detail.
- Test the real, live address. Testing a local copy or a staging site can give very different numbers. When we test our own pages on a local machine, the test overstates the page weight because the HTML isn’t compressed the way it is on the live HTTPS site.
- Try it on your own phone with Wi-Fi off. Open the site on mobile data, ideally on an older phone, and time how long it takes before you can read the main text.
- Run it more than once. Lab scores move a few points between runs. Look at the band (good, needs work, poor), not the exact number.
Core Web Vitals in plain English
Google measures loading experience with three Core Web Vitals. Each one has a “good” threshold, assessed on 75% of real visits:
| Metric | What it measures | Good |
|---|---|---|
| Largest Contentful Paint (LCP) | How long until the main content, usually the big image or heading, appears | 2.5 seconds or less |
| Interaction to Next Paint (INP) | How quickly the page responds when someone taps a button, opens a menu or types | 200 milliseconds or less |
| Cumulative Layout Shift (CLS) | How much the page jumps around while loading, such as text moving when an image or banner appears | 0.1 or less |
There are two kinds of data. Field data comes from real Chrome users visiting your site, and is what Google uses; small sites often don’t have enough visitors for it to show. Lab data comes from a simulated test like ours, which is available for any site and is the best way to find the causes.
The usual causes, most common first
1. Oversized images
The most common cause by far. A photo straight from a phone or a stock site can be several megabytes and thousands of pixels wide, displayed in a box a few hundred pixels across. Every visitor downloads the full file. Hero images and sliders at the top of the page hurt LCP the most.
2. Too many plugins and page builders
On WordPress, each plugin can add its own scripts and styles to every page, whether that page uses them or not. Visual page builders add a lot of code of their own. Ten or twenty plugins is common, and together they’re often the difference between a fast site and a slow one.
3. Cheap or overloaded hosting
On low-cost shared hosting, your site shares a server with many others. If the server is slow to respond, everything else waits. A slow server response time in the test report points here.
4. Third-party scripts
Chat widgets, booking embeds, review widgets, social feeds, video players, several analytics and advertising tags: each one loads code from someone else’s server. They’re the usual cause of poor INP, because they keep the phone’s processor busy when the visitor taps something.
5. Things that jump around
Images without set dimensions, cookie banners that push content down, adverts and late-loading fonts all cause layout shift. Visitors tap the wrong thing, which is exactly what CLS measures.
6. No caching or compression
Without caching, the server rebuilds each page on every visit. Without compression, text files travel at full size. Both are usually a setting, not a project.
7. Video backgrounds and big sliders
They look good on a large screen and cost a lot on a phone. A single strong image usually does the same job.
The fixes, in order of payoff
- Resize and compress images. Serve images at the size they’re displayed, in a modern format such as WebP or AVIF, and lazy-load the ones below the first screen. Don’t lazy-load the main image at the top; that delays LCP.
- Remove what you don’t use. Deactivate and delete unused plugins, remove widgets that don’t earn their place, and limit third-party scripts to the ones you’d notice missing.
- Turn on caching and compression. Most good hosts and WordPress caching plugins handle both.
- Set image and video dimensions and reserve space for banners, so nothing moves while loading.
- Load fonts sensibly. Use one or two font families, host them on your own site and let text show straight away.
- Upgrade the hosting if the server response is still slow after the steps above.
Do them in this order and test after each one, so you know what actually helped.
When patching stops working
Sometimes a site has been patched for years: a heavy theme, a page builder, twenty plugins and three tracking tools. Each fix brings a small gain and the next update takes it away. At that point a rebuild is often cheaper than another round of speed work.
That’s why we build custom sites by default. Every page is generated in advance as finished HTML, with no database and no plugins running when a visitor arrives, and our own sites built this way score 98–100 in Google’s Lighthouse test on mobile. When a client needs WordPress, we keep the theme and plugins lean. The trade-offs are explained in our comparison of custom-built websites and WordPress.
Speed also matters for businesses where the visitor is in a hurry, such as restaurants and cafés or a cleaning company someone is ringing between other jobs.
How B63 helps
Speed optimisation is part of every site we build, and on our monthly plan (€0 upfront, then €150 per month excl. VAT for 24 months) it continues for the whole term, alongside hosting, updates, backups, changes and professional SEO. Sites slow down as content is added, so keeping them fast is ongoing work; it’s one of the routine tasks on our maintenance checklist. More on our SEO and page speed page.
FAQ
Does website speed affect Google rankings?
Page experience, including Core Web Vitals, is one of many things Google looks at. It rarely outweighs relevant, useful content, but a slow site also loses visitors before they read anything, which matters just as much.
My site is fast on my computer. Why is it slow for customers?
Your computer is more powerful than most phones, your office connection is faster than mobile data, and your browser has probably cached the site already. Test on a phone, on mobile data, in a private window.
What is a good PageSpeed score?
90–100 is good, 50–89 needs work, and below 50 is likely costing you visitors. Treat the score as a guide to what to fix, not a goal in itself.
Will a faster hosting plan fix everything?
Only if the server is the bottleneck. If the page is heavy with images and scripts, faster hosting helps a little and the phone still has to download and run everything.
Want to see where your site stands? Run the free website check, or send us the address and we’ll tell you what we’d fix first.