Why is my website slow? A diagnostic checklist

Most slow websites are slow for one of five reasons: oversized images, too much JavaScript, no caching, a slow server response, or third-party scripts. Work through them in that order, because the first two account for the majority of the problem on most sites.

Start with images

Images are almost always the largest thing on a page, and almost always the easiest fix. Serve modern formats, size them to the space they actually occupy, and lazy-load anything below the fold.

A photograph exported straight from a camera or a stock library can be three megabytes. Properly compressed, the same image at the size it displays is usually under two hundred kilobytes with no visible difference.

Then cut the JavaScript

Every script has to be downloaded, parsed and executed before the page becomes interactive. Sliders, chat widgets, analytics, heat mapping and social embeds each add weight, and they compete with each other on the main thread.

Audit what is loading and be ruthless. If a widget does not directly help a visitor decide to buy or enquire, it is costing you more than it returns.

Check caching and server response

Caching means the server stops rebuilding the same page for every visitor. Without it, a content management system runs database queries and renders templates on every single request.

Server response time should be under 600 milliseconds. If it is not, the cause is usually shared resources under contention, an old PHP version, or a database that has never been optimised. Guaranteed CPU and RAM on a private shared plan solves the first one immediately.

Measure, do not guess

Use field data rather than a single lab test. A one-off score tells you how the page performed once, on one connection, from one location. What matters is how it performs for your actual visitors on their actual phones, which is what Core Web Vitals field data reports.