Yesterday’s patch seems to have fixed this.
Currently, these numbers are how many Iron Bar’s can be made with a single Oil:
5.4 Furnace (55 sec)
6.8 Improved Furnace (68 sec)
11.9 Kiln (84 sec)
27.2 Fuel-Efficient Furnace (192 sec)
5.3 Heat-Efficient Furnace (11 sec)
The easy-to-make level 55 Fuel-Efficient Furnace still beats out the rare recipe Kiln, but otherwise these numbers make a lot more sense for their effort.
PS - All 5 furnace’s have the same Ironstone to Iron Bar ratio. So it comes down to how much fuel you want to use vs how fast the you want to smelt things.