Profiling your site is extremely useful not only for debugging and performance testing but also for learning about how your site handles requests.

Symfony 2 comes with a bundle (like a Drupal module but for Symfony) called WebProfiler Bundle which collects information about each request made to your application and allows you to visualize it in the browser.

Drupal 8 doesn’t include the Web Profiler bundle in core. However, there already is a contrib module for it. Make sure to install any dependencies before enabling the module.

The module will render a fixed bar at the bottom of each page for users with the View webprofiler toolbar permission. The bar provides a summary of the collected data from the profiler.