drupal as mobile app backend
Not sure if "faster" but I'd recommend the integration of Drupal Commerce and Phonegap (I'm guessing you're using DrupalGap already) if you're already using both: https://www.drupal.org/project/commerce_drupalgap

You've got plenty of resources there to get started such as the Commerce Services module.

It's up to you to decide where you need or not a full bootstrap of your backend, I'm thinking you need it to get advantage of user access, permissions and consistency check in your order status, but it depends a lot on your app and where  you're implementing the business logic. If you access the database directly, without any bootstrap is going to be faster, but the same way you build a drupal site, if you bypass Drupal to access directly to the database that'd be faster as well.