On the Additional Options page of J.E.D.I. there are two thumbnail options that impact the zip file size and import processing time. Here is a little bit of information to help choose the best configuration for your theme.
This option determines whether or not the existing thumbnails of the demo site are included in the zip file packaged by J.E.D.I.
This results in larger zip files, but faster processing time on import. This option is enabled by default because most server configurations allow for a larger zip file than a longer processing time.
When this option is disabled, then just the original image is included in the zip file packaged by J.E.D.I.
This results in smaller zip files, but can also result in server timeouts because WordPress will then create the thumbnails during the import process. That can take a while with Divi because there are several thumbnail sizes set by Divi.
Whether or not this option is enabled, the number of images and size of the images play a big part in the size/speed of import. For that reason it is best to have fully optimized images for the best success rate.
The way this option works is it tells WordPress that there are no thumbnail sizes at all for the import process. After the import completes, thumbnail options are returned to their normal state. This allows the import process to be much faster and avoid server timeouts, especially when Media Thumbnails are not included in the Export File.
This will allow the images to be imported, but it can confuse users because then the media library looks like it is full of blank images. The images are actually there, there are just no thumbnails to show in the library. On the front end the Divi Modules will default to showing the full size image when it would normally use a smaller thumbnail version. Since it is supposed to be demo content it isn’t terrible for the full image to be used even in smaller places. When users upload new images to replace the demo images then thumbnails will be created.
This option is disabled by default, because it is more confusing for users. If they want thumbnails, they could run a plugin to regenerate thumbnails.