Is using nodes for a notification system a performance concern?
I would go with Messaging. Dedicated Entity (if implement properly) is always faster than Node. I participated in the big D7 project and we were happy with Message API. By using a custom entity you can switch to custom storage, like: https://www.drupal.org/project/mongo_entity

This can be helpful for D8:

https://drupalwatchdog.com/blog/2015/3/entity-storage-drupal-8-way