Magento is robust and requires layers of caching and proper optimization to be performant. You can read more about some suggestions at: https://variux.com/magento-2-caching-technology/
That notwithstanding, with proper hosting, caching, and development, Magento can scale to fit the needs of enterprises.
Not all eCommerce platforms have the features or capabilities to handle the unique needs of large organizations.
Is there a particular platform that you had in mind? There are many dozens of eCommerce platforms on the market. It's hard to compare Magento to everything at once.
Even so, I believe it will come down to a unique use-case. If you need the unique features and benefits of Magento, including available extensions and integrations, you'll go with it. If you can get everything you need out of a smaller / cheaper solution, that's what you'll do.
It's analogous to WordPress and Drupal. Drupal has some selling points that make sense for bigger organizations. That doesn't mean that it's for everyone, but it will continue to have a strong following even if it takes more work to launch on average based upon the way it's been architected.
