old:
      if pixAlpha >= alpha_ignore:
        for colour_id in filtered_colours_to_filter:
          ..

new:
  pixcache = dict()    // before y loop

      if pixAlpha >= alpha_ignore:
        winnerID = pixcache.get(pix)
        if not winnerID:
          for colour_id in filtered_colours_to_filter:
            ..
        pixcache[pix] = winnerID;