When new content such as pages or assets are created via the API or through the out of the box import job it is adding rows to tblNotificationSubscription. This only happens when a project has been added. In the client's case they run a custom sync job which "creates categories, products, variants, location pages, and images". They are not programatically adding the new content to a project, it is happening automatically. The additional rows in tblNotificationSubscription impact performance when loading the CMS UI due to the loading of netNotificationSubscriptionListByKey.
It should be noted that I have only replicated the behavior with the out of the box import job. The customer's sync job is using the service API to create variants.
COM-12010 created to address service API.