Upgrade to the CDN module!
The Parallel CSS - AdvAgg Plugin module is now obsolete.
The CDN module contains a superset of the Parallel CSS - AdvAgg Plugin module's functionality, with a better UI, and a Drupal 7 port readily available.
No migration path is provided, but it's very simple, see #1410318: Merge into CDN module?. More details are available in the CDN module's Advanced Help documentation. Install CDN module version 2.5 or later, which ships with #1452092: Automatically distribute certain filetypes over multiple CDNs.
Below you, can find a quick tutorial on how you can duplicate the Parallel CSS - AdvAgg Plugin module's behavior using the CDN module.
Why did/does this module exist?
Modern browsers cap the simultaneous image download rate at somewhere around 6 images per domain. This module allows your site to pull images from separate domains simultaneously, thereby removing this cap, and allowing your site to load images more quickly.
This also holds the potential to free up server use, or at least divert some of the requests to different servers so that you do not reach your sever request limits as easily.
In other words: this allows your admins to manage the source URLs your images are referencing during the CSS aggregation process.
Old set-up (pre-CDN module):
At admin/settings/advagg/parallel-css
, enter the domains: