Using the Drupal Batch API

function mymodule_setup_batch($start=1, $stop=100000) {
  //  ...
  //  Populate $lots_of_data from record $start to record $stop.
  //  ...
 
  //Break up all of our data so each process does not time out.
  $chunks = array_chunk($lots_of_data, 20);
  $operations = array();
  $count_chunks = count($chunks);
 
  //for every chunk, assign some method to run on that chunk of data
  foreach ($chunks as $chunk) {
    $i++;
    $operations[] = array("mymodule_method_to_work_on_a_small_part", array( $chunk ,'details'=> t('(Importing chunk @chunk  of  @count)', array('@chunk '=>$i, '@count'=>$count_chunks))));
    $operations[] = array("mymodule_another_method",array($chunk));
  }
 
  //put all that information into our batch array
  $batch = array(
    'operations' => $operations,
    'title' => t('Import batch'),
    'init_message' => t('Initializing'),
    'error_message' => t('An error occurred'),
    'finished' => 'mymodule_finished_method'
  );
 
  //Get the batch process all ready!
  batch_set($batch);
  $batch =& batch_get();
 
  //Because we are doing this on the back-end, we set progressive to false.
  $batch['progressive'] = FALSE;
 
  //Start processing the batch operations.
  drush_backend_batch_process();
}

function mymodule_method_to_work_on_a_small_part ($chunk, $operation_details, &$context) {
  //Do something to $chunk, maybe create a node?
  $context['message'] = $operation_details; //Will show what chunk we're on.
}
function mymodule_another_method($chunk, &$context) {
  //Do some more work.
  $context['message'] = t('We have done a second thing to a chunk of data');
}

function mymodule_finished_method($success, $results, $operations) {
  //Let the user know we have finished!
  print t('Finished importing!');
}

function mymodule_drush_command() {
  $items  = array();
  $items['myimport'] = array(
    'callback'    => 'mymodule_setup_batch',
    'description' => dt('Import'),
    'arguments'   => array(
      'start'     => "start",
      'stop'      => "stop",
    ),
  );
  return $items;
}
 
function mymodule_drush_help($section) {
  switch ($section) {
    case 'drush:myimport':
      return dt("import items from the Internal Database [start record] [end record].");
  }
}