High speed descents do not consume any extra fuel, provided you are at flight Idle from TOD to the same given point in space
I would beg to differ. If you plan a high speed descent your TOD point obviously moves closer to your destination resulting in longer spent with the engines cruise fuel flows. A slow speed descent can mean up to 5 minutes more time with engines at idle resulting in a fuel saving which is why very low cost indexes produce slow descent speeds.