In previous versions of Drupal (drupal 6,7), some features (page caching and CSS aggregation) were turned off on a new Drupal installation and required an administrator to enable them.

With Drupal 8 these performance-enhancing features are enabled by default, which encourages best practices, and helps to ensure that more Drupal sites take advantage of them.

Drupal 8 is fast by default, by having default settings that improve speed: CSS and JS aggregation are enabled, JavaScript assets are loaded from the bottom of the page , whenever possible, block caching is enabled by default, and so on.