FastFurnace utilizes mixins to edit the functionality of the Furnace, Blast Furnace, and Smoker, improving their TPS performance (not smelting speed!)
Similar to FastWorkbench, it caches the last recipe used, and checks this recipe first before re-scanning the entire registry. This is significantly faster due to the vanilla furnace scanning the entire furnace registry each tick, which may be fine for vanilla, but in a modded environment, gets very large very quickly.
These changes apply to the Furnace, Blast Furnace, and Smoker. Any mod utilizing AbstractFurnaceTileEntity will also receive the benefits of having this mod installed.
For people who like numbers, here is Vanilla vs FastFurnace. This is 9 furnaces each smelting cobblestone the entire time the test was conducted. The number of invocations is kept roughly the same as to not skew any numbers. This test was conducted in vanilla, which only has ~70 furnace recipes. In a modpack, there could be upwards of 700, significantly increasing the performance gain.
Be sure to also check out FastWorkbench & Toast Control!