Generator speeds up other buildings that craft. It would help to have that said somewhere in game, and by how much since I don't know.
Thanks a lot for the great feedback! This is insanely useful!
Here is the building efficiency math:
Base efficiency: 100%
Blacksmithing skill: +20% (Forges & Furnaces)
Engineering skill: +30%
Generator buff: +180% (doesn't stack with other Generators)
So the maximum possible efficiency for Forges/Furnaces is 330% and for every other building is 310%.
Another cool thing to note is that Generators also increase Mining Rod's attack speed!