As you found out, using AForge.NET is a good idea (you just have to add it as a nuget). I suggest you use its Median filter which is often used for denoising (see Median Filter in wikipedia).

AForge needs 24bpp RGB images, so you need to convert it first in your sample case, but here is an example of code that seems to work quite well on it:

  // load the file as 24bpp RGB
  using (var bmp = LoadForFiltering(@"C:\temp\Testing-Image3.tif"))
  {
      var filter = new Median();

      // run the filter 
      filter.ApplyInPlace(bmp);

      // save the file back (here, I used png as the output format)
      bmp.Save(@"C:\temp\Testing-Image3.png");
  }


  private static Bitmap LoadForFiltering(string filePath)
  {
      var bmp = (Bitmap)Bitmap.FromFile(filePath);
      if (bmp.PixelFormat == PixelFormat.Format24bppRgb)
          return bmp;

      try
      {
          // from AForge's sample code
          if (bmp.PixelFormat == PixelFormat.Format16bppGrayScale || Bitmap.GetPixelFormatSize(bmp.PixelFormat) > 32)
              throw new NotSupportedException("Unsupported image format");

          return AForge.Imaging.Image.Clone(bmp, PixelFormat.Format24bppRgb);
      }
      finally
      {
          bmp.Dispose();
      }
  }

If you really need high performance, then you could go for NVidia CUDA/NPP (using directly the GPU) for example but this is more work, not directly supported from C# (and requires an NVidia card of course). Related question here: 2D CUDA median filter optimization and a white paper on CUDA here : Image Processing and Video Algorithms with CUDA