Batch writes blocks rendering

[antoniozzo]

Describe your environment

• Operating System version: macOS 10.14.6
• Browser version: Chrome 79.0.3945.117
• Firebase SDK version: 7.6.2
• Firebase Product: firestore

Describe the problem

When I'm updating a few hundred documents with batches, rendering is blocked while firestore is sending the changed documents back through the WebChannel. Having firebase.firestore.setLogLevel("debug") on I can see that I get a bunch of messages, one for each document change: [Connection]: WebChannel received: {"documentChange": .... While these messages are received the rendering of my application is blocked.

Using snapshot.docChanges() I have already got the documents and rendered the changes, but the lag happens after that, while the server is responding with pending writes.

[thebrianchen]

@antoniozzo Thanks for filing an issue! We're currently working on improving the performance of Firestore. While Firestore is processing the documents, the rendering of your application will unfortunately be blocked. However, if you update your documents in smaller batches, there will be gaps in between batches for you to render, even though the total amount of processing time required by Firestore is still the same.

Another option is to offload Firestore onto a web worker, which will free up your main thread while Firestore is processing documents received over WebChannel. The caveat is that persistence currently does not work when Firestore is run on web workers (though we currently have an experimental build trying to test that out).