In the snapshot release it was stated that ancient debris is best smelted in a blast furnace, as it takes longer to process. Testing shows that it takes exactly the same time as iron ore in both a furnace and blast furnace.

Steps to reproduce:

1. Place two furnaces and two blast furnaces in a row.
2. Attach hoppers to the backs of said furnaces (furnii?).
3. Place redstone dust over the hoppers and to a lever, flip lever on to lock the hoppers.
4. place 1 coal in each locked hopper.
5. Place 16 ancient debris in a furnace, 16 ancient debris in a blast furnace, 16 iron ore in a furnace, and 16 iron ore in a blast furnace.
6. Turn off lever, watch furnaces process and complete.