Consider updating the WASAPI code to use a newer API which supports lower latency.

From noel@bandlab.com, it might be possible to use WASAPI's low-latency shared mode to reduce the latency.  Something like:

/ Initialize the shared mode client for minimal delay.
  HRESULT hr = client->Initialize(AUDCLNT_SHAREMODE_SHARED,
                                  stream_flags,
                                  0,
                                  0,
                                  reinterpret_cast<const WAVEFORMATEX*>(format),
                                  session_guid);
  if (FAILED(hr)) {
    DVLOG(1) << "IAudioClient::Initialize: " << std::hex << hr;
    return hr;
  }

  if (use_event) {
    hr = client->SetEventHandle(event_handle);
    if (FAILED(hr)) {
      DVLOG(1) << "IAudioClient::SetEventHandle: " << std::hex << hr;
      return hr;
    }
  }

  UINT32 buffer_size_in_frames = 0;
  hr = client->GetBufferSize(&buffer_size_in_frames);
  if (FAILED(hr)) {
    DVLOG(1) << "IAudioClient::GetBufferSize: " << std::hex << hr;
    return hr;
  }