Advanced CSS/JS Aggregation

AdvAgg allows you to improve the front-end performance of your site. Be sure to do a before and after comparison by using Google's PageSpeed Insights and WebPagetest.org. The performance benefits are achieved by using some of the features found in AdvAgg and its sub modules. Out of the box AdvAgg's front-end performance will be similar to core's.

https://www.drupal.org/project/advagg

OSTraining has a great two video series on How to Use AdvAgg