Performance issues with `AVAssetWriter`

Hey all!

I'm trying to build a Camera app that records Video and Audio buffers (AVCaptureVideoDataOutput and AVCaptureAudioDataOutput) to an mp4/mov file using AVAssetWriter.

When creating the Recording Session, I noticed that it blocks for around 5-7 seconds before starting the recording, so I dug deeper to find out why.

This is how I create my AVAssetWriter:

let assetWriter = try AVAssetWriter(outputURL: tempURL, fileType: .mov)
let videoWriter = self.createVideoWriter(...)
assetWriter.add(videoWriter)
let audioWriter = self.createAudioWriter(...)
assetWriter.add(audioWriter)
assetWriter.startWriting()

There's two slow parts here in that code:

1. The createAudioWriter(...) function takes ages!

   This is how I create the audio AVAssetWriterInput:

   // audioOutput is my AVCaptureAudioDataOutput, audioInput is the microphone
   let settings = audioOutput.recommendedAudioSettingsForAssetWriter(writingTo: .mov)
   let format = audioInput.device.activeFormat.formatDescription
   
   let audioWriter = AVAssetWriterInput(mediaType: .audio,
                                        outputSettings: settings,
                                        sourceFormatHint: format)
   audioWriter.expectsMediaDataInRealTime = true
   

   The above code takes up to 3000ms on an iPhone 11 Pro!

   When I remove the recommended settings and just pass nil as outputSettings:

   audioWriter = AVAssetWriterInput(mediaType: .audio,
                                    outputSettings: nil)
   audioWriter.expectsMediaDataInRealTime = true
   

   ...It initializes almost instantly - something like 30 to 50ms.

2. Starting the AVAssetWriter takes ages!

   Calling this method:

   assetWriter.startWriting()
   

   ...takes takes 3000 to 5000ms on my iPhone 11 Pro!

Does anyone have any ideas why this is so slow? Am I doing something wrong?

It feels like passing nil as the outputSettings is not a good idea, and recommendedAudioSettingsForAssetWriter should be the way to go, but 3 seconds initialization time is not acceptable.

Here's the full code: RecordingSession.swift from react-native-vision-camera. This gets called from here.

I'd appreciate any help, thanks!