300000+ records user importing problem
For an import of that size you will need to run the import in the background and in steps.

This will use cron to run the feeds cron job. Drupal uses a poor-mans cron so it would be best to setup a cron on the server or use a module like Elysia cron. If you do not not you could try something link https://www.pingdom.com (you can use a free account) to ping your site every e.g. minute to trigger the drupal corn.

I would also make sure that your server is optimal for Drupal 7 and just for the import consider increasing php memory limit and execution timeout value.

Nevertheless I would recommend using a more generic module like "Migrate", though it needs some more work to be done.

I personally imported 23K user profiles into Drupal a few months ago, so it is possible.