Most slow websites are slow for one of five reasons: images that are far bigger than they need to be, cheap or overcrowded hosting, too many plugins, a heavy theme or page builder, and caching that’s missing or misconfigured. Often it’s two or three at once. The good news is that each one can be measured, and the fixes are well understood. A good target is for the main content of each page to appear within 2.5 seconds on a mobile phone.
Speed matters because visitors leave slow pages, and because Google uses page experience signals, including the Core Web Vitals below, when it ranks results. Here’s how to work out what’s slowing your site down, starting with how to measure it.
What is a good page load time?
Aim for the main content of a page to load within 2.5 seconds on mobile. That’s Google’s “good” threshold for Largest Contentful Paint, the moment the biggest visible element (usually a hero image or headline) finishes appearing.
“Page load time” used to mean the moment everything on the page had finished downloading. That number is less useful now, because a page can be perfectly usable long before the last tracking script loads. What visitors notice is how quickly they can see the content, how quickly the page reacts when they tap something, and whether things jump around while it loads.
What are Core Web Vitals?
Core Web Vitals are three measurements Google uses to judge how a page feels to real visitors:
- Largest Contentful Paint (LCP): how long the main content takes to appear. Good is 2.5 seconds or less.
- Interaction to Next Paint (INP): how quickly the page responds when someone taps or clicks. Good is 200 milliseconds or less.
- Cumulative Layout Shift (CLS): how much the layout jumps while loading, like a button moving just as you go to press it. Good is 0.1 or less.
You can check all three for free. Put a page into Google PageSpeed Insights, or open the Core Web Vitals report in Google Search Console, which groups your pages by how real visitors on Chrome experienced them. Test on mobile. Google indexes the mobile version of your site, and mobile visitors on a patchy signal are the ones who notice a slow page first.
Why is my website so slow? The five usual causes
To make this practical, take a hypothetical site. Say you run a garden design business in Surrey. Your WordPress homepage opens with a full-width photo of your best project, there’s a gallery further down, a page builder theme, about 30 plugins, and hosting that cost a few pounds a month. PageSpeed Insights gives the homepage an LCP of 6 seconds on mobile. Here’s what we’d look at, in the order we’d look at it.
1. Images that are too big
This is the most common cause by far. The hero photo came straight from the photographer at 6,000 pixels wide and 5MB. On a phone it displays at about 400 pixels wide, so the visitor downloads more than ten times the data they need.
The fix:
- Resize images to the largest size they’ll actually be shown at.
- Save them in a modern format such as WebP or AVIF, which are usually much smaller than JPEG at the same quality.
- Serve different sizes to different screens, so phones get smaller files. WordPress does much of this automatically if images are added properly.
- Lazy-load images further down the page, so the gallery only downloads when someone scrolls to it. Never lazy-load the hero image, which needs to appear first.
2. Hosting that can’t keep up
Cheap shared hosting puts your site on a server with many others, all competing for the same processing power. When a neighbour gets busy, your site slows down. You’ll see this in PageSpeed Insights as a slow “server response time” (often labelled Time to First Byte).
Does hosting affect website speed? Yes, directly. No amount of image tuning fixes a server that takes two seconds to start responding. Better hosting, with server-level caching and a content delivery network (CDN) that serves files from locations near your visitors, is often the biggest single improvement on a slow WordPress site.
3. Too many plugins, or the wrong ones
The number of plugins matters less than what they do. One badly built plugin that loads its scripts and styles on every page can cost more than ten well-built ones. Sliders, social feeds, chat widgets and “all-in-one” toolkits are frequent culprits.
Go through the list and ask of each one: do we still use this, and does it need to load on every page? Delete what you don’t use. Replace heavy plugins with lighter alternatives or a few lines of custom code.
4. A heavy theme or page builder
Multipurpose themes and page builders are built to do everything, so they ship code for features you’ll never use. The result is large stylesheets and scripts that the browser has to download and process before it can show the page, which also hurts INP on slower phones.
The long-term fix is a lean theme built around what your site actually needs. In the short term, many builders have performance settings that stop unused code loading, and they’re worth switching on.
5. Missing or misconfigured caching
Without caching, WordPress builds every page from scratch for every visitor: it queries the database, runs the plugins and assembles the HTML. Page caching saves the finished page and serves the saved copy instead. Browser caching tells returning visitors’ browsers to keep files like logos and stylesheets rather than downloading them again.
Good managed hosting handles page caching at server level. If yours doesn’t, a well-configured caching plugin helps, but set it up carefully: pages with forms, baskets or logged-in content need excluding.
Measure first, then fix the biggest problem. Most slow sites get most of their speed back from two or three changes, and the report tells you which ones.
How do I speed up a WordPress website?
Work through the causes in the order above, testing after each change:
- Run your homepage and one key service page through PageSpeed Insights on mobile, and note LCP, INP and CLS.
- Compress and resize the largest images, starting with whatever the report names as the LCP element.
- Check server response time. If it’s slow, move to better hosting before anything else.
- Remove unused plugins and test whether any single plugin is making a big difference.
- Turn on page caching and a CDN.
- Set width and height on images, embeds and ads so the layout doesn’t jump, which fixes most CLS problems.
Back up before you start, and make changes on a staging copy where you can. Speed work is also part of routine upkeep, because sites tend to slow down again as plugins and content pile up. Our WordPress maintenance checklist includes a regular speed check for that reason.
How we approach website speed
Slow sites often have a hosting problem and an image problem at the same time, so we look at both. Our WordPress care plans run on Kinsta’s managed WordPress hosting on Google Cloud with Cloudflare in front, and include performance monitoring and ongoing optimisation, so speed gets checked regularly once the site is fixed. When the theme itself is the bottleneck, our in-house team builds lean custom themes as part of web design.
FAQs
Why is my website slow on mobile but fast on desktop?
Phones have less processing power than laptops and often run on slower mobile connections, so large images and heavy scripts hurt far more. Your desktop test may also be using a fast office connection and a cached copy of the site. Always test on mobile in PageSpeed Insights, because that’s the version Google uses for indexing.
What is a good PageSpeed Insights score?
A score of 90 or above is rated good. The score is a lab test, so it varies from run to run; the Core Web Vitals figures from real visitors, shown at the top of the report when your site has enough traffic, are the better guide to what people actually experience.
Does website speed affect SEO?
Yes, though it’s one factor among many. Google uses Core Web Vitals as part of its page experience signals, and a very slow site loses visitors before they engage, which hurts enquiries whatever your rankings. Relevance and content quality still matter more than shaving the last half second off a fast page.
Will a caching plugin fix a slow website?
It often helps, especially on hosting without server-level caching, but it won’t fix oversized images, a slow server or a bloated theme. Treat caching as one fix among several, and test pages with forms or baskets after turning it on.
