justread | Minimal web content beautification with Readability | Awesome List library

Assuming in my app I've already read an entire collection with a snapshot listener.

If a document is added several seconds after an entire collection has been read does it triggers an entire collection read? or just the new document?

For example - a chat app between 2 people:

a collection (represents a chatroom) contains 4 documents (each represents a message) is already been read by a user, hence 4 reads. if the person on the other side sends another message, does this mean another 5 reads just went underway (4 old document, and a brand new one), resulting in a total of 9 reads? or only the new collection is been read, resulting in a total of 5 reads (4 from the beginning and another after the listener detected a new document inserted to the collection)?

Just to be clear all of the procedures described in the example (from the initial read) takes several seconds.

I can't find a solution or a similar question online, and I cant understand if in the firebase documentation there is an answer to it no matter how much I search there.

EDIT WITH SOMEWHAT OF AN ANSWER:\

After trying to figure out the exact numbers, I've run a test that resulted the following:

a collection with (say) 20 document (that a listener is attached to), that you add another 10 documents result in way more than 10 reads.

My conclusions for chat like implementations I would recommend using Firebase realtime database and not firestore. With a childeventListener you can extract and read only new messages without the need to re-read several models that you've already pre-loaded.

EDIT CODE I'VE RUN TO TEST: