MigrateBatchExecutable rewinds the iterator each batch iteration

Rewinding the iterator indeed a major performance issue on large data sets, but it can be circumvented by enabling [high water mark]( https://www.drupal.org/node/1223936 → ) check or adding its support to your custom source plugin based on [SqlBase::initializeIterator()](https://git.drupalcode.org/project/drupal/-/blob/11.x/core/modules/migra...).

Here is a nice article about the topic: [Speed up Your Drupal Migrations with Highwater Marks](https://drupalize.me/blog/speed-your-drupal-migrations-high-water-marks).