If you change the format of your source data, then your mapping needs to updated for that format. Usually as simple as changing the format setting.
To bulk import such large numbers you’ll need to do it as a background process (might need to turn this on in setup.php), otherwise the browser or web server will just timeout on the connection. The webserver timeout can be increased, but only if you have control over the server and its not a shared hosting
If it seems to always stop progressing at a certain number, it might be something wrong with your source data, so check the logs for errors, as some can cause php errors that aren’t shown on screen.
