Furnaces need to be rebalanced. I tested Improved, Kiln, Fuel-Efficient, and Heat-Efficient using 100 branches each as fuel and making bricks to see how fast they went through the fuel and how many bricks they ended up with when they ran out of branches.
Heat-Efficient was definitely the fastest and wound up with 62 bricks.
Improved and Kiln cook bricks at the same speed, but Kiln goes through fuel 20% faster, so it only made 80.
Improved and Fuel-Efficient both made 100 bricks, but Fuel-Efficient took much longer to do so.
So if you want to sacrifice fuel for speed, use Heat-Efficient, otherwise the Improved Furnace is king with better fuel efficiency (and equal speed) than the Kiln and better cooking speed (and equal efficiency) than the Fuel-Efficient.