How to write image to ffmpeg with multi thread

I'was trying to encode video from opencvSharp mat to ffmpeg. Now,It's.done. But,It has low speed encoding problem.

for (int  StartFrame = 0; StartFrame < EndFrame; StartFrame ++)
{                  
    using (var ms = new MemoryStream())
    {     
        //read mat from videocapture
        Mat mat =CreateRenderingMat(StartFrame)
        BitmapConverter.ToBitmap(mat).Save(ms, ImageFormat.Bmp);
        ms.WriteTo(proc.StandardInput.BaseStream);
        Cv2.WaitKey();
        mat.Dispose();
        ms.Dispose();
        ms.Close();
        Debug.WriteLine(movePos.ToString());
     }
 }

Now, I wanna manage it with multi threading. But,I don't make sence. Tell me some advice. Thank you.