As often as possible is not working in periodic option

Yes, the import period depends on cron.

I see that this isn't mentioned on the form, while the D7 of Feeds did mention it. Attached a patch to fix that.

You could write a shell script that runs every x minutes and in there run items from the queue "feeds_feed_refresh".

Example:

use Drupal\Core\Queue\RequeueException;
use Drupal\Core\Queue\SuspendQueueException;

/**
 * Number of seconds to run tasks from the "feeds_feed_refresh" queue.
 *
 * @var int
 */
const FEEDS_FEED_REFRESH_TIME = 60;

$queue_worker = \Drupal::service('plugin.manager.queue_worker')->createInstance('feeds_feed_refresh');
$end = time() + FEEDS_FEED_REFRESH_TIME;
$queue = $queue_factory->get($queue_name);
$lease_time = FEEDS_FEED_REFRESH_TIME;
while (time() < $end && ($item = $queue->claimItem($lease_time))) {
  try {
    if ($item->created > Drupal::time()->getRequestTime()) {
      // Item was postponed! Release the item and ontinue to the next one.
      $queue->releaseItem($item);
      continue;
    }

    $queue_worker->processItem($item->data);
    $queue->deleteItem($item);
  }
  catch (RequeueException $e) {
    // The worker requested the task be immediately requeued.
    $queue->releaseItem($item);
  }
  catch (SuspendQueueException $e) {
    // If the worker indicates there is a problem with the whole queue,
    // release the item and the queue.
    $queue->releaseItem($item);

    watchdog_exception('cron', $e);

    // Stop the loop.
    break;
  }
  catch (\Exception $e) {
    // In case of any other kind of exception, log it and leave the item
    // in the queue to be processed again later.
    watchdog_exception('cron', $e);
  }
}

Tests are passing for the patch, so committed #2.

Oh wait, in a shell script you could also use the Drush command 'queue-run'. Is a lot simpler.

drush queue-run feeds_feed_refresh

https://drushcommands.com/drush-9x/queue/queue:run/

Do the instructions on the following page help?
https://www.webfoobar.com/index.php/node/25

From the example there, instead of drush @site1 cron, your command would be drush @site1 queue-run feeds_feed_refresh.

The code presented there would go into a .crontab file.

Either way, you need to be able to run cron jobs to run imports regularly. With the method described above you can avoid running cron for all modules.

After posting I just realized you'll get more control about which module's cron to run with the Ultimate Cron module.

On crontab you would configure to run regular cron every minute and with Ultimate Cron you can configure in the Drupal user interface per module and per queue how often they may run.

I haven't tried the module for D8 yet, but the D7 version worked great.

@John_sm
Ah, I forgot the queue "feeds_feed_refresh" is a derivative, which means in this case that a queue plugin is generated for every feed type. So the actual queue would be called something like "feeds_feed_refresh:my_feed_type" where "my_feed_type" is the machine name of your feed type.

drush @site1 queue-run feeds_feed_refresh:my_feed_type

Secondly, the queues can be empty, because they require feeds_cron() to be called in order to schedule imports. So it looks like your command would also need to call that function:

drush @site1 php:eval "feeds_cron();"

@John_sm
The queue name is feeds_feed_refresh: plus the feed type ID.