That makes sense. Regardless of the actual speed of item processing, I do think it should state how soon the next one will be available, as I think it only says when the entire batch will be done right now.
The idea of a slider (literally or in difficulty level choices) is interesting. I understand some people like being able to adjust every aspect of the game manually, which is fair. Personally, I like things done a bit more organically and in-game. Just brainstorming a few ideas...
Do you think having the ability to craft fancier furnaces that process items faster (and more of them in a single batch) would help that? It might be slow-going at first, as a lot of these sorts of games always are in the beginning, but as you work more and upgrade your workshop and associated crafting stations, you become faster and more productive. Perhaps even having the ability to increase the speed of production by expending more power stones, or some other uncommon resource?
I suggested in a Steam comment that maybe the player can choose different paths to focus their time on; organically, instead of "pick your profession" is far more fun for me personally - the more time you spend doing something, the more it levels up, the more efficient you become at doing that thing - but I imagine some people like the idea of having to pick one thing to focus on to get more out of repeat playthroughs. If something like this were implemented, then it would make sense for things like production speed to go up and become faster as you level that skill up, if you're focusing on the workshop (other "paths" could be things like spelunking, fighting, fishing, farming, etc).