[2021] MOVED: some forges smelting faster than others?

That is definitely odd.. The only thing I can think is that you had access to the Advanced Engineering perk when you placed the faster two forges, but not when you placed the slower two... But iirc the 20% buff from Advanced Engineering should be dynamic..

Did you place two of the forges and someone else place the other two? Or is this in Singleplayer? And do you have Rank 1 Advanced Engineering, Or did you re-spec out of it via a fergettin elixir?