You could split your image into tiles and do some parallel processing by creating a custom transformer.

Converting to the polygons can be quite slow as you detail. But you could also go out to points and rasterise the points based on the cell spacing of the input raster cells.

I did it slightly different with this example, utilsing a pointcloud approach and extracting white (255,255,255). Sorry, you get a picture of me from almost 10 years ago, but you will get what I am trying to do.