Hi, Tom from the Firebase Realtime database team here. To address the difficulties of figuring out where performance issues were, we made a database profiler (https://firebase.google.com/docs/database/web/profile). I expect that would help pinpoint where the issue was. My informed guess would be an index was not setup as you expected and the querying was being done client side, or my next guess would be the client was rapidly connecting and disconnecting. (check clientside with Firebase.database.enableLogging(true)).

The profiler excludes SSL overhead, but you can see usage inclusive of SSL in the webconsole.