Preparse responsive images
I've managed to get it working via the following steps:


Create a new field and give it the preparse field type
Duplicate only the twig variable code from the template into the input box in the field page.
In Craft Settings, go to the page template, and add the new preparse field to the available fields (this doesn't show up in the content editor).
The twig template stayed the same.
The responsive images are then generated when the user saves the entry in Craft instead of when the page is loaded by the end user.