Memory consumption issues in Composer builds.
Consider adding the composer optimizations project, to remove legacy Symfony builds with:
composer require zaporylie/composer-drupal-optimizations
See more information about the project: https://github.com/zaporylie/composer-drupal-optimizations
and Check out Jeff Geerlings' blog post https://www.jeffgeerling.com/blog/2018/make-composer-operations-drupal-way-faster-and-easier-on-ram