By default, the Drupal 7 core loads around 20 different CSS and Javascript files.

So, in order to make your Drupal site run quickly, it's essential to load those files as quickly as possible. It's easy to get started by enabling the aggregation and compression options in Drupal.

Aggregation gathers all the files together and loads them as one. Compression reduces the size of each individual file, stripping out unnecessary characters, whitespace and implementing CSS shortcuts.

In this video, Robert Ring shows you how enable Drupal's core aggregation and compression options: