Are you on a local server or a hosting?
Try to disable the generation of images during the import with the following code. You can add this code to the end of functions.php file in your theme.
add_filter(
'pt-ocdi/regenerate_thumbnails_in_content_import',
'__return_false'
);
You can try to import the content manually, with the original WordPress importer plugin. The XML is located in /booklium/assets/demo-data/booklium-%theme-variant%