I generally choose to use captives to hurry production of buildings rather than settling them as slave specialists or selling them for commerce. However if a city can build a specific building in one turn, the captive cannot be used to hurry it. Which is quite annoying if the city has a high production and can build most buildings in one turn. Sometimes there are ways around this: if you remove all your population from the city map, and turn the citizens (which give 1 hammer each) into a type of specialist that does not give hammers, some buildings might be buildable in 2 turns instead of one. And then you can use the captive to hurry production. Then, after using the captive, you have to change the population back to how it was. But this is annoying micromanagement. Please make it so that a captive can always hurry production even if the building only needs one more hammer to be completed.