Game mode: [Online | Official US server #1941]
I have tested numerous conditions using in each test separately and a timer.
I have used a lvl 2.3. and a legendary smelter in the standard furnace smelting for each test, brick, hardened brick, iron, steel and hardened steel.
Each test showed no determinable difference in time reduction regardless of the level of Thrall smelting the items.
It is also unclear from the developers if smelters reduce costs, although we simply know from experience that they do not however they should decrease the time it takes the higher the level of Thrall, because, to put it simply, then whats the point of using them at all?