Firestore query single document

In a flutter app, I need to pull the users document from firestore only if the last updated field is greater then the last updated locally.

Currently this is what I do:

    QuerySnapshot usersData = await Firestore.instance
        .collection("users")
        .where("lastUpdated", isGreaterThan: lastUpdatedLocally)
        .where("userId", isEqualTo: userId)
        .getDocuments();
    Map<String, dynamic> userData =
        usersData.documents.isEmpty ? {} : usersData.documents.first.data;

I have three questions:

1. Is it possible to do something like this in a get query for a single document, it doesn’t seem right to query the whole collection knowing that the document id is the user id.
2. If not, can we do a query by the document id, something like: .where(<documentId>, isEqualTo: userId) instead of having a filed in the document called "userId".
3. And lastly, is it slower to do what I did here then to query a single document (if it is possible).

DocumentSnapshot snapshot = await Firestore.instance.collection("users").document(userId).get()

var document = await Firestore.instance.collection('COLLECTION_NAME').document('TESTID1');
document.get() => then(function(document) {
  print(document("name"));
});