So, not quite what you asked for but there are some tricks to speed up fusion.
The biggest one if that if you mix multiple types of items around the same level it'll cut down fuse times... 2 different items cuts fuse times in half, 3 types brings it down to a third, etc. So if have have multiple items you need around similar levels you can mix them in the cauldron as "fuse buddies" to speed it up some. You can also try more creative fuses (like CF or 4F1E) to cut down on the number of fuses needed - though this does add randomness to the results.
Anyway, back to the initial suggestion, even with mixed fuses some things still take days so I wouldn't mind seeing times shortened a little there (mostly on 45+ items is when it gets ridiculous). But I'm not too sure about letting us speed it up with a building, I feel like that might be a little too powerful given how important fusion is and how cheap gold is.
Also just curious, if we got a building with an instant boost to reduce fuse times, what would the building itself be for? I'm having trouble thinking of anything it could unlock as it levels up ^^;