Aggregate CSS and JS files in Drupal core

You can make Drupal perform better by aggregating CSS and JavaScript files. This will save bandwidth and server requests, resulting in faster load times for the users.

Enable caching, and CSS and JavaScript aggregation

• Go to Configuration > Development > Performance and set "Browser and proxy cache maximum age" to a time span which works well.
• Under Bandwidth Optimization enable "Aggregate and compress CSS files", and "Aggregate JavaScript files".

Note that enabling "Aggregate JavaScript files" may cause redeclaration in rare cases. The workaround is to disable aggregation for the problematic JavaScript library:

mymodule.somelibrary:
  js:
    js/foo.js: { preprocess: false }