You are doing an upgrade that takes 2 weeks to finish. Now you can spend a crazy amount of gems to finish, or you can be patient and wait it out, both fine options. What about a third option to assign a second builder to assist with the upgrade?? Now this wont be a cost free service to take advantage of, but rather cost some gems and maybe extra resources to do.
A building takes 2 weeks to build and you just started it. You want to add a second worker that costs (xxx) gems to do (and maybe extra resources). This second builder reduces upgrade time by X%. Of course the times reduction would also need to be worked out as well. Now players can accelerate build time while still using gems at the cost of tying up 2 builders on a building.
