Hard to say without more context. Depends on what is actually causing the delay. Would something like this work?
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
if([_captureSession isRunning])[_captureSession stopRunning];
dispatch_async(dispatch_get_main_queue(), ^{
[self.bgImageView setHidden:NO];
[self.insideView setHidden:NO];
[self.scanView setHidden:YES];
[self.previewLayer removeFromSuperlayer];
});
AVCaptureInput* input = [_captureSession.inputs objectAtIndex:0];
[_captureSession removeInput:input];
AVCaptureVideoDataOutput* output = (AVCaptureVideoDataOutput*)[_captureSession.outputs objectAtIndex:0];
[_captureSession removeOutput:output];
});