What is the most common cause of a slow website?

The most common causes in order of frequency are: (1) Large unoptimised images, (2) No browser caching configured, (3) Slow hosting/high TTFB, (4) Render-blocking JavaScript, (5) No CDN. Fixing these five covers the majority of slow website cases.

My website was fast and now it is slow — what changed?

Common causes of sudden slowdowns are: a new plugin or script added, theme or platform update, increased traffic overwhelming shared hosting, server hardware issues, a large image added to a template affecting all pages, or a CDN configuration change.

Should I fix everything in the performance report?

Prioritise by impact. Start with the "Opportunities" section showing the largest potential savings in seconds or kilobytes. Fix the highest-impact issues first and re-test. Some minor "Diagnostics" items have negligible real-world impact and are not worth extensive effort.

Does switching to a faster host really make a significant difference?

Yes — for many sites, hosting is the single biggest bottleneck. Migrating from shared hosting to a managed WordPress host or VPS can reduce TTFB from 1.5 seconds to 100–200ms, delivering the biggest speed improvement of any single change.

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Slow Website Diagnosis: What's Slowing Your Site Down?

A slow website is not just one problem — it is usually several layered issues. This guide walks through the systematic diagnosis of slow websites, from server-level problems to browser rendering bottlenecks, with clear explanations of what causes each issue and how to fix it.

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Step-by-step diagnosis methodology

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Covers all main causes: server, images, JavaScript, CSS

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Identifies priority fixes for maximum impact

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Run the Website Speed Test first to identify which metrics are failing — then use this guide to diagnose the root cause of each issue.

How It Works

Step-by-step guide to slow website diagnosis: what's slowing your site down?:

1
Run the speed test and note all failing metrics
Run the Website Speed Test on your page. Note every metric in the red/failing range — LCP, CLS, TTFB, TBT, and any resource-level opportunities.
2
Diagnose server speed (TTFB)
If TTFB is over 600ms, investigate hosting (upgrade plan or switch to managed hosting), database query performance, server-side caching, and CDN configuration.
3
Diagnose image issues
Check the resource list for large images (over 200KB each). Compress all images, convert to WebP, and implement srcset for responsive images.
4
Diagnose JavaScript and CSS blocking
Check for render-blocking resources. Add defer/async to non-critical scripts. Minify and combine CSS files. Remove unused CSS with tools like PurgeCSS.

Real-world examples

Common situations where this approach makes a real difference:

E-commerce site sudden slowdown

An online store that was previously fast suddenly slows to 8-second load times after a marketing team adds a new personalisation platform. TTFB diagnosis identifies the new platform is adding 4 seconds of server-side processing. Switching to client-side personalisation restores previous performance.

WordPress site audit

A WordPress site with a score of 38 on mobile is diagnosed systematically: TTFB 1.8 seconds (shared hosting — switch to managed WP hosting), images uncompressed (+3MB — install Imagify), 12 render-blocking scripts (install script manager plugin). Each fix is applied in order and re-tested.

New developer inheritance

A developer inheriting an old codebase uses the speed test and this diagnosis guide to systematically identify and document all performance issues before presenting an improvement roadmap to the client.

When to use this guide

Use this guide when your website is noticeably slow, when you have received a failing performance score, or when you want to systematically work through all possible causes of poor speed.

Pro tips

Get better results with these expert suggestions:

Start with TTFB — slow servers explain a lot

Time to First Byte (TTFB) measures how long it takes the server to start sending data. A TTFB over 600ms usually points to slow hosting, uncached database queries, or an inefficient back-end. This is the most impactful single fix for many sites.

Images are the most common culprit

Over 60% of a typical website's payload is images. Uncompressed images, missing next-gen formats (WebP/AVIF), and oversized images (serving desktop images to mobile) are the most common and most fixable speed problems.

JavaScript blocks rendering — defer what you can

Any script in the <head> without async or defer blocks the browser from rendering the page until that script has downloaded and executed. Deferring non-critical scripts can often reduce render blocking time by 1–3 seconds on mobile.