Nanite Factory produces nano-sized robots, which are considered the ultimate achievement in Robotic technology. These are capable of greatly increasing construction speed of buildings, defenses and ships. In game terms, the Nanite Factory halves the construction time for all buildings, ships, and defense cumulatively for every level that is upgraded. Thus it is the only building which, assuming you don't change your Robotics level past the level 10 requirement, always takes the same time to upgrade (~55h).