@Daryl - all importers need more love....

As for the WP importer IMO there should be two variants, the current one to be used for exporting from local PC and an API based for big sites. You just can't import a big site to a shared host because of file upload limitations, memory limitations and execution time limitations, which usually are not under the control of the site owner.

wp cron is not the right tool in any case because the site owner will like to see the import progress and check the site when it is finished. This needs to be done with an ajax request to trigger the next batch of import when the previous finishes.