For WordPress there are many plugins and themes. Unfortunately, their focus is often not on performance. This plugin helps you to identify the slow components of your website.
Most performance breaks in WordPress can be identified very well and quickly in the front end, but sometimes it is necessary to look under the hood to see what slows down the build of the WordPress installation. The Plugin Query Monitor can be of great help.
What can Query Monitor do?
Query Monitor is a developer tool for WordPress that allows you to debug database queries, PHP errors, hooks and actions, gutters, inserted scripts and stylesheets, HTTP API calls, and more. It includes advanced features such as debugging Ajax or REST API calls, and can also narrow down much of the output to plugins or themes, so you can quickly determine if plugins, themes, or features are the culprits.
The plugin extends the admin toolbar with the basic debugging information regarding the respective subpage. When the menu is clicked, the panel opens at the bottom with grouped and very detailed information.

WordPress: Query Monitor has identified the break.
The screenshot above is from my website where there were some performance drops in the backend. In this particular case, it is easy to see that the Essential Grid plugin has PHP errors which is one of the reasons for the slow performance.
In addition, the plugin also provides information about the PHP environment, the database, WordPress and the web server.