Furnaces are boring. You pop something in. Then you wait a bit. Then they're done. They are too quickly done to do something in the meantime, but take long enough to be a noticeable wait. Staring at a screen whilst waiting for things is not fun. You want to play! My suggestion is to make furnaces interactive in some way. Perhaps a mini-game that is similar to the fishing mini-game. Keep it short, keep it simple, but make it interactive! (Perhaps a perfect game gives you better quality yields?)
I feel like you maybe haven't smelted higher level ores, because "too quick to do something in the meantime" really only applies to copper, and maybe iron. Smelting e.g. iridium takes most of an entire day. Copper can be smelted while you wait and watch, but gold and iridium are very much "throw 'em in the furnace in the morning and get them at noon/evening". At least that's how I do it because it takes long enough to not be worth hanging around for.
I'm someone that uses 10 furnaces in the mine so it might be difficult for me to play the game even if I was an octopus.
Iridium and gold take a long time to melt, but the need for bar will be significantly higher later on that mini games like that wouldn't be necessary.
Oh, OK. Thanks for the feedback! I'm rather new to the game, and just noticed that smelting was a boring chore at this level. Nice to hear it being less tedious at higher level. I hadn't thought about that. Very good point! I guess I retract my suggestion. :]