The Developer tool looks inside the site: its PHP error log, hooks, REST API routes and database tables. It’s for developers and for anyone a developer is helping.
Go to All-In-One Utilities → Tools → Developer. Only administrators can see it (super admins on multisite), since it shows the site’s internals. Everything is read-only except emptying the error log.
Error Log
Reads the site’s PHP error log, most recent first. Identical messages are grouped, with how many Times they happened and when they were Last seen. Filter by level (such as warnings or fatal errors) or by text, such as a plugin’s folder name. Only the most recent 512 KB of a big log is read.
Empty the log clears it, so you can reproduce a problem and see only the new messages.
No log?
WordPress writes the log only when WP_DEBUG and WP_DEBUG_LOG are on in wp-config.php. Turn them off again on a live site when you’re done.
Hooks
Every callback attached to a hook, with its priority, file and line. Hooks with the most callbacks shows where most of the work happens.
REST API
Every REST API route, grouped by namespace. Routes anyone can call are flagged Public, which is right for public content but worth checking for anything else.
Database
Lists the tables with their size and row count. Open a table to browse its rows.
Cron Events → opens the Cron Events tab of Site Performance.
See also Site Performance check and Code snippets.
Get the add-on: compare the plans for All-in-One Utilities Pro, then pick the one that fits your sites.