RPScreenRecorder startCaptureWithHandler quality

I'm trying out screen recording in my app and I'm testing two different methods. I'm testing both with just recording a draggable element being quickly moved around the screen. Video quality produced by startRecordingWithHandler is great. The dragged element moves around smoothly and there's virtually no difference between the video and how it originally looked. I need more control over the recording, though, so I was excited to see that startCaptureWithHandler was added in iOS 11, but the quality is just atrocious. The video is really choppy and you just see the dragged element occasionally jumping around the screen. I've tried various video settings for the asset writer but I would expect much better from the following basic high bitrate settings:

@{
  AVVideoCodecKey: AVVideoCodecTypeH264,
  AVVideoWidthKey: width,
  AVVideoHeightKey: height,
  AVVideoCompressionPropertiesKey : @{
  AVVideoAverageBitRateKey : [NSNumber numberWithInt:20000000],
  },
  };

Has anyone else tried startCaptureWithHandler and gotten decent results?