Why does calling AVCapturePhotoOutput.capturePhoto freeze preview(AVCaptureVideoDataOutput.captureOutput)

I am using AVCaptureVideoDataOutput to capture each frame from camera capture.

I display these frame to MTKView.

that's fine.

If I call AVCapturePhotoOutput.capturePhoto, the delegate function AVCaptureVideoDataOutput.captureOutput will be paused, so that my preview had a little freezed, that's very bad.

I don't know if I do something wrong cause this?

this is my capturePhoto code:

Code Block 
func createTakePhotoSettingForNormalPhoto() -> AVCapturePhotoSettings {
    var settings: AVCapturePhotoSettings
    settings.isHighResolutionPhotoEnabled = true
    return settings
}
func takePhoto() {
    settings = createTakePhotoSettingForNormalPhoto()
    settings.flashMode = ...
    _outputStillImage.capturePhoto(with: settings, delegate: self)
}

I noticed that if I tap shot button repeatly and quickly in IOS native Camera APP, it will have a black flash at each shot, but the preview also could display frame smoothly.