Here's the new summary:
The build time tooltip math was fixed in rP19966, so now we can consider the GUI part.
The production queue shows a countdown timer since rP19657, and I think the same would be useful here.
{F196310}
Here's the old summary:
Currently when constructing a building, the time to completion is displayed in a tooltip on the health bar, shown in the first picture below. Meanwhile, the time speed-up (how much time we'd save if we added another builder) is displayed in a tooltip on the hammer icon, shown in the second picture below.
{F146586}
I think it would make more sense to show these two numbers on the same tooltip, shown in the third picture above.
The formulas used for the times are wrong (they only work if you don't have any buffed units, like an elephant or a man with the +10% building rate from "female inspiration"), and I'm fixing the math in D522. But the GUI is a separate consideration, so I thought I should offer a separate patch for that. Of course I can combine the two if both are approved.
Also, it might be nice to make the tooltip slightly wider (there's plenty of room, my screenshot is cropped), so that the "This foundation..." string always fits on one line. Actually, it seems like using "The foundation..." instead of "This foundation..." does the trick. So maybe we can leave that alone.