Are Firebase queries a scalable

In my app to check for if an email (and username) is taken when signing up I use queries like this...

let emailRef = Firebase(url: "https://photocliq5622144.firebaseio.com/users")

emailRef.queryOrderedByChild("email").queryEqualToValue(email.text!.lowercaseString).observeEventType(.Value, withBlock: { email in

Would this work well with hundreds or even thousands of users (is this scalable)?