The base recipe is 2 Lead ore make 1.666 lead plates on average. The Vulcanite based recipe is 20 Lead ore make 6 Plates, which is equal to 2 Lead ore make 0.6 Plates. This is considerably worse efficiency, when it actually should improve efficiency.
I would suggest to change the recipe to 8 Lead ore and 1 Vulcanite block make 10 Lead plates in 24 seconds base crafting time. This mirrors the usual 50% productivity gain from using Vulcanite assisted smelting.