Dispatch_async in captureOutput:(AVCaptureOutput*)captureOutput didOutputSampleBuffer, EXC_BAD_ACCESS error on sample buffer

I am trying to get the sample buffer from captureOutput:(AVCaptureOutput*)captureOutput didOutputSampleBuffer, process it and then append it to an AVAssetWriter. The whole code works, however it gets really slow and I get low fps on older devices.
I thought of putting it inside a dispatch_async to improve the performance, which however leads to an EXC_BAD_ACCESS error as soon as the sample buffer is accessed.
How could I fix it, while keeping the code in background?

queue1 = dispatch_queue_create("testqueue", DISPATCH_QUEUE_SERIAL);
...

-(void) captureOutput:(AVCaptureOutput*)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection*)connection{
    
dispatch_async(queue1, ^{
...
if(captureOutput == videoOutput){
//I process the buffer by appending an image to an adaptor
if([writerVideoInput isReadyForMoreMediaData] && recordingAssetWriter.status == 1)
    [adaptor appendPixelBuffer:pxBuffer withPresentationTime:CMSampleBufferGetPresentationTimeStamp(sampleBuffer)]) //<-- here I get EXC_BAD_ACCESS
}
if(captureOutput == audioOutput){
...
// I then append the audio buffer
if([assetWriterAudioInput isReadyForMoreMediaData] && recordingAssetWriter.status == 1)
    [assetWriterAudioInput appendSampleBuffer:sampleBuffer];
...
}
});

}

From the captureOutput:didOutputSampleBuffer:fromConnection: discussion in the AVCaptureVideoDataOutput.h header file:

Clients that need to reference the CMSampleBuffer object outside of the scope of this method must CFRetain it and then CFRelease it when they are finished with it.

So it looks like you need retain those sample buffers because they're going out of scope! Don't forget to release them later, otherwise you'll leak a lot of memory.

I had forgotten that ARC in Objective-C does not manage CoreFoundation objects.

The header file then goes on to warn against holding on to the sample buffers for too long lest you drop frames.