In certain versions of ImageMagick there is a problem when trying to import the demo images and the system becomes extremely slow. This causes the demo importer to never complete. You can use a workaround to fix this as follows:
Access the root of your WordPress installation (located in the same folder as your /wp-content/ and /wp-admin/ directories) using an FTP client or file manager in your web hosting control panel where your .htaccess is located. If you can’t find it, then look at this article on why you can’t find the .htaccess file and how to find it.
Next, add this line to your .htaccess file:
SetEnv MAGICK_THREAD_LIMIT 1
This code limits the number of cores using MAGICK_THREAD_LIMIT which is used when uploading the demo images.