Contao ships with the symfony/profile-pack package that includes a Web Developer Toolbar and the Profiler view to use as a powerful development tool.

More information about the general Symfony Profiler can be found in the Symfony Documentation.

However, Contao provides a custom data collector that collects and processes specific data from Contao requests, and presents them in the profiler. Furthermore there is also a Web Developer Toolbar extension that looks like this.

The data collectors, profiler and web developer toolbar are usually only enabled in dev mode. As soon as the mode is changed to prod, those features are disabled for performance and security reasons.

You can access the profiler directly via /_profiler/.