The bug

When cooking in a furnace, wooden slabs provide the same amount of fuel as planks. This means you can double your output for free just by crafting slabs. In fact, this renders charcoal completely worthless, as one log can be crafted into enough slabs to smelt 12 items, where that same log would only smelt 8 if it was made into charcoal.

How to reproduce

1. Place two furnaces
2. Put a stack of ore in the top of each
3. Put a wooden plank in one, and a slab in the other
4. Wait
→ They finish at the same time

Expected
Slabs should have exactly half the cook time as planks