Which import tool are you using exactly? The one found under “Tools” in WordPress itself? The challenge there is that imports that take a very long time can cause the hosting service to time out. In that case, you have the following options:
- Check with your hosting provider to see if you can increase the timeout limits. Depending on the size of the import, this might help – but it might not be enough.
- Don’t use the browser-based import, as that causes timeouts; instead, use the WP CLI: https://developer.wordpress.org/cli/commands/import/ – you’ll need SSH access to the hosting for this. If necessary, contact your hosting provider about this as well.
- Or use a different plugin for the import. There are many to choose from that can also bypass any timeouts that might occur: https://wordpress.org/plugins/tags/importer/
I’m using the “built in” one in the Tools section.
So, what did the tests of my tips mentioned above reveal?
I haven’t had time to work on it yet. I’m trying something else, breaking up the posts into years and importing them one group at a time to see if that is faster/easier.