71 sec to perform a GraphQL query ("noMatchingRole": 913047)

Hello to all, my name is Luca from Italy.
I’m quite new with MongoDB Realm and maybe my question is trivial.
I have a collection with more or less 1 millions Document.
As test, I created a new property user_id on 6 Documents, and set its value with a user Id (same user used in Postman to perform the GraphQL query … auth username/password)
I also created an index on the user_id property in MongoDB Atlas.

When i perform a simple query to get all Documents of that user, the respond time is 71 sec and in the log I see:

Rule Performance Metrics:
{
  "namespacesMetrics": {
    "gl-ms-events.evt_events": {
      "roles": {
        "sync": {
          "matchingDocuments": 0,
          "evaluatedFields": 0,
          "discardedFields": 0
        }
      },
      "noMatchingRole": 913047
    }
  }
}

Could you please help me to understand where the problem is ?