VNDetectFaceLandmarksRequest lags when used with AVCaptureVideoDataOutput

Question

I am using a

VNDetectFaceLandmarksRequest

combined with a

VNSequenceRequestHandler

to process images coming from the

AVCaptureVideoDataOutput

delegate call:

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection)

I want to be able to show the face landmarks in realtime, so that the user can see them overlaid over the AVCaptureVideoPreviewLayer.

So, as the AVCaptureVideoDataOutput delegate gets called, I then call:

func perform(_ requests: [VNRequest], on image: CIImage)

The CIImage comes from the CMSampleBuffer the delegate provides.

The problem is that the landmarks lag. As I move my face in front of the camera the landmarks are quite slow to update.

How can I update the landmarks so that they follow the face movement without lagging behind?