When right-clicking furnace, add a 'Smelt Previous' option which will automatically start smelting the same bars (or cannonballs) you smelted last time. This same idea could apply to other workstations like pottery wheels, spinning wheels, anvils etc.
This will speed up the smelting process by eliminating the need to select the bar you intend to smelt. You already know what you want to smelt so spending time to select the bar each time is unnecessary. Currently you can just hold space bar as the game remembers what you selected last time, but this would make it more streamlined.
This would benefit players who are creating large volumes of the same item, which would be the majority of players. It would be particularly helpful for mobile users, and for clients with a menu-entry-swapper plugin.
The next time you go smithing or bowstring spinning, just think about how much simpler and more comfortable it would be to only have to click the workstation once.
Thank you for reading!