When your website is slow, whether WordPress or otherwise, there are a few techniques you can use to help identify the culprit.
In developing a website there will undoubtedly be changes along the way. Some will install new themes, add plugins, embed video, chat and social media plus an array of other features designed to improve the user experience so that it's more engaging in an effort to build an audience and sell more stuff.
Check your page loading times before making changes, and again after, so that you understand exactly what's making things go slow.
Not only will PageSpeed Insights show how long a page takes to load completely, but also how long to get key parts of the page loaded and what can be done to speed things up.
Another tool that not only measures speed but gives information about improvement opportunities, GTmetrix can be used to establish benchmarks and retest after changes are made.
Providing you with the means of picking what type of devices, network speed and location to test from, WebPage Test is another top tool for measuring speed and diagnosing performance concerns.
If you're using a CMS, ecommerce package or other web app that uses themes to manage appearance, then the theme swapping technique can be used for diagnosing performance problems.
With this technique, site performance is measured using the current theme, then the themes are swapped onto one that is light, default or known to be high performance, with performance retested and speeds compared to see how the primary site theme scores.
Themes are a frequent source of performance degradation which makes this quick test approach a popular option along the road to improving speeds.
In addition to swapping themes, plugins are a prime candidate for performance concerns, providing of course your website relies on an application platform that uses plugins, like blogs or ecommerce packages.
The process is very straightforward. Run a benchmark speed test on a few pages. Disable a plugin and run the tests again. Compare the differences. Then re-enable the plugin and move on to the next one.
This is very close to the "swap 'til you drop" approach of replacing parts until you find the broken one. It's time consuming. It's imperfect. It's also likely to help you find out exactly where the slowdown lies, if a plugin is at fault.
For sites built on WordPress, there are a handful of handy helpers (plugins) available to assist in diagnosing what's happening with performance.
Checking for compatibility, updates, that files haven't been tampered with and is configured without errors, the Health Check and Troubleshooting plugin gives valuable insights about the site performance.
There are times when a plugin uses the WordPress database and adds so much weight that it slows down the performance of regular operations. Plugins Garbage Collector will give information about the non-core database tables.
For debugging issues in WordPress including queries, PHP errors, scripts, styles, blocks (and more), Query Monitor is a great diagnostic tool for helping to find what is slowing a site, as well as throwing errors on a site. When you want to know what plugin is slowing down a site, it's a great starting point in diagnosing.
Knowing how much resources are consumed by various pages on a WordPress site, UsageDD gives valuable intel about numbers of queries, RAM consumption and more - a truly helpful tool in diagnosing performance issues.
Want to know if your install is healthy and fully updated? Then WP Health is for you! Dozens of checks in the free version make it worth the effort. When you want even more, there's a premium version that adds ongoing monitoring and notifications.
This page is dedicated to assisting you in diagnosing performance problems. If you'd like to know more about web performance optimization, be sure to check out our resources on this wiki, the forums and our blog.