create-menu-items.php

<?php

// Create a menu.
$values = array(
  'id' => 'menu-test',
  'label' => 'Test menu',
  'description' => 'Description text',
);
\Drupal::entityTypeManager()->getStorage('menu')->create($values)->save();


// Create 300 menu items.
$storage = \Drupal::entityTypeManager()->getStorage('menu_link_content');
$base_options = array(
  'title' => 'Menu link test',
  'provider' => 'menu_link_content',
  'bundle' => 'menu_link_content',
  'menu_name' => 'menu-test',
);
$values = $base_options + array(
    'link' => ['uri' => 'internal:/menu-test/hierarchy/parent'],
  );
for ($i = 0; $i <300; $i++) {
  $values['title'] = 'Menu link test' . $i;
  $storage->create($values)->save();
}