Yeah, for overall performance it could be true that it may double every two years, especially as the amount of people starting to seriously work with 3d printing seems to be exponentially increasing. But performance here does not equal speed, I see it more as a combination of all factors, so then overall performance would remain a subjective quality. For some it's more in aesthetic representation of the design intent, for other it's more to do with mechanical properties, for others speed and profitability is most important. I don't see the speed doubling every two years since that will depend a lot on the motors and materials used, and those don't evolve as fast as electrical components.