Fastest way to convert image to Black and white image

LEADTOOLS toolkits can help you do this easily (Disclaimer: I work for LEAD).

You can specify a region and then apply image processing operations on that region without affecting the remaining pixel data of the image. You can find more about specifying regions here:
https://www.leadtools.com/help/leadtools/v19/dh/to/leadtools.topics~leadtools.topics.creatingaregion.html

About converting the pixel data to black and white, you can use LEADTOOLS functions like IntensityDetectCommand Class.

IntensityDetectCommand _intensityDetectCommand = new IntensityDetectCommand(130, 255, RasterColor.White, RasterColor.Black, IntensityDetectCommandFlags.Master);
void _viewer_MouseMove(object sender, MouseEventArgs e)
{
if (e.Button == System.Windows.Forms.MouseButtons.Left)
{
_viewer.Image.AddEllipseToRegion(null, new LeadRect(e.X - 30, e.Y - 30, 60, 60), RasterRegionCombineMode.Set);
_intensityDetectCommand.Run(_viewer.Image);
_viewer.Image.MakeRegionEmpty();
}
_viewer.Refresh();
var g = _viewer.CreateGraphics();
g.DrawEllipse(new Pen(Color.Green, 3), new Rectangle(e.X - 30, e.Y - 30, 60, 60));
g.Dispose();
The result will look like this: