Debug your WordPress site using Query Monitor

Query Monitor is a free WordPress plugin which provides concise insights about the performance of your site’s pages. It offers an in-browser dashboard which allows you to debug database queries, view PHP errors, see plugin/theme contributions, view API calls, and more.

How does Query Monitor help me improve my site?

The Query Monitor plugin is a great debugging tool for helping to determine the root cause of performance issues, namely those caused by themes, plugins, external services, and other site-level issues which might need to be addressed.

Alongside this, the plugin also gives insights into the server’s contribution to performance, making it a good tool to check if you believe the server is running into limitations as well.

Using Query Monitor

First things first, be sure to install Query Monitor and activate it on your site.

Accessing the dashboard

Once the plugin is operational, a new block of numbers will appear within the WordPress admin bar at the top of the page. Hovering over them will give you a dropdown list of various sections within Query Monitor, while clicking on any of them will bring up the Query Monitor auditing window itself (which will dock to the bottom of your browser window by default).

Overview tab

Once inside Query Monitor’s dashboard, the default view will be of the Overview section, which outlines the page’s performance at a glance.

The main three areas to be aware of are:

1. Page Generation Time, or how long it took for the page itself to get created.
2. Peak Memory Usage, which is the highest amount of memory used while creating this page. You always want the Usage number to be lower than the Limit number.
3. Database Query Time, which is how long the database itself took to respond to requests from the theme/plugins. This number will be the ‘total’ of all individual contributors within the “Query” section, so if this is a low number, then plugins and themes are likely not related.

Queries tab

The next most useful section is under the Queries tab, specifically the Queries by Component area. It breaks down the contributions to the page load by source, so it’s a great place to begin looking for slowness coming within the site itself.
You’ll be able to see Core (which is WordPress itself), as well as the names of each individual plugin and theme. You can click into each of the contributors to see more detailed information as well.

PHP Errors tab

Another handy tool within Query Monitor is its ability to point you to PHP Errors, Warnings, and Notices to help debug potential issues within the theme/plugin code. If any are present, a new tab will appear named PHP Errors, which shows the “level” of severity, as well as what message the issue came along with and where the issue occurred.
The location of the issue helps to inform you of which area of the site the problem stemmed from. In the example above, the issue occurred in a theme named “twentytwenty” within the “footer.php” file on line 60.

HTTP API Calls tab

The final tab this article will cover is the HTTP API Calls tab, which outlines any sort of POST/AJAX call which happened on the page. These can be useful in helping to determine if dynamic content on the page is holding up the page load at all.
This is most commonly seen with theme or plugins which rely on third-party services to provide information to the page, or if there are long-lived calls to the server itself to fetch new information to display. This can be a great tool for helping to determine if slow calls are the culprit behind a page’s poor performance.

Need help?

If you have any questions our Happiness Engineers are here to help!

If you discover problems within Query Monitor itself, you can reach out to Query Monitor’s support team directly as well.