Try this. Look at one of your cities that has recently reached the 12K population mark. Pick one of the tier 4 buildings you haven't constructed yet. Right click on it in the construction window to see how long it will take. As an example, I think a dockyard takes 5 turns to build. Whichever one it is, it will have a number over an hourglass in one of the top corners; that is the number of turns it is supposed to take. Now queue it up, and look at the construction time by holding your mouse pointer over the building in the queue. If the building normally would take 5 or more turns to construct, the actual time will be one turn less.