Iron Tower's Oscar has put together the first installment of a multi-part feature that examines the methods the team has taken to improve the performance of The Age of Decadence, and what the overall results have been. It's tough to quote anything from it since a vast majority of the text is dependent on the embedded screenshots, but I'll at least share his conclusion:
Until now, all the performance increases were done keeping the visual quality of the game. Disabling shadows nets a big increase (from 78 to 135 FPS in our game), and reducing or removing the groundcover also nets some gains. On top of that, there are the different options offered by the engine like lower texture resolution, lower lighting system (for low end machines), among other things.
Also, there are ways to increase the visual quality of the visuals. These shots were taken with the default options, using a 1024x768 resolution, no anisotropic filtering, so there's room for more visual candy.
Now, we are working on improving performance with a lot of characters on screen which is quite crappy at the moment. My next update would cover that plus some nice additions like texture tinting.