My web application performs the following Firebase Firestore query:

<script defer src="/__/firebase/9.6.5/firebase-app-compat.js"></script>
<script defer src="/__/firebase/9.6.5/firebase-firestore-compat.js"></script>
<script defer src="/__/firebase/9.6.5/firebase-storage-compat.js"></script>
<script defer src="/__/firebase/9.6.5/firebase-analytics-compat.js"></script>
<script defer src="/__/firebase/init.js?useEmulator=false"></script>
<script>
      document.addEventListener('DOMContentLoaded', function() {
        firebase.firestore().collection("users").where("username", "==", username).limit(1)
            .get()
            .then((querySnapshot) => {
                //…
            })
            .catch((error) => {
                //…
            });
      });
</script>

The single query results in 3 network calls:

The first call is the query request written explicitly in my code snippet. The third call contains “removeTarget”:2 in the payload.

My biggest concern is this second call below that takes 1 minute to execute, sometimes being several Kilobytes in size:

https://firestore.googleapis.com/google.firestore.v1.Firestore/Listen/channel
?database=projects%2F<omitted>%2Fdatabases%2F(default)
&gsessionid=<omitted>
&VER=8
&RID=<omitted>
&SID=<omitted>
&CI=0
&AID=0
&TYPE=xmlhttp
&zx=<omitted>
&t=1

The GET response was an empty 200 with the following duration:

What are these second and third calls? I am especially interested in what the second call is given the duration and size of the call.