A simple Console plugin that could run the source set thumbnails generation on the background.

• Download the plugin from: the packages folder
• Install the plugin and make sure the Responsive plugin is already installed and enabled (configured).
• Enable the plugin from the System->Plugins, using the filters to select the type: Console.
• Using SSH or a cron job run php www/cli/joomla.php responsive:build, assuming that your public folder is named www (if not adjust the command to reflect the correct path).
• That's it, observe PHP doing a recursive generation of different sizes of each image in all the folders specified in the plugin Filesystem::Local!