Just leaving this here for next time I need this. The change below allows you to change the number of items that Drupal’s feeds importer will process at once.

/* Set feeds importer to import in chunks of 500, rather than 50
 * Ref: http://drupalcode.org/project/feeds.git/blob/b39b547768abbda3b0d7b5425593c0e7c05800f2:/README.txt#l185
 */
$conf['feeds_process_limit'] = 500;