As explained at Why you should run migrations in Drush rather than the UI, drush is a faster and more robust means of running large migration operations than the web-based UI is. As of Migrate 2.6, it is possible (with some advanced server-side configuration) to start migration operations from the UI that will run in the background via drush on the server.
Configuring background operations
To enable background imports and rollbacks requires some configuration on the server side. The key element is letting the Migrate module know where the drush command resides on the server, so it can be run. This is done by setting the Drupal migrate_drush_path variable to the full path of the drush command on the server, either in settings.php:
$conf['migrate_drush_path'] = '/usr/bin/drush';
or by setting the variable using drush:
$ drush vset migrate_drush_path /usr/bin/drush
On a windows server the path of the drush command has to include '
drush.bat', e.g. like