When it comes to importing demo content, before you hit import, disable all 3rd party plugins (to free up server resources), then go to import, open chrome developer tools, hit Import, and watch console for errors. When some error occurs refresh import page and repeats the process. Importing demo content must reach 100% without displaying error in the console. So repeat it until the console is clear (sometimes you have to repeat the process for 3-4 times or maybe more).
Please understand that this is a common issue when importing demo with all theme authors since demos become more robust so they can satisfy needs of all customers, so the issue is usually triggered by lower server resources. Usually, it is due to low WordPress memory limit https://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP and also due to low PHP directives, so it would be good if you can increase these values or ask your hosting service to do that for you:
max_execution_time = 300;
memory_limit = 128M;
upload_max_filesize = 128M;
post_max_size = 256M;
Helpful reading – http://www.wpbeginner.com/wp-tutorials/how-to-increase-the-maximum-file-upload-size-in-wordpress/