From David Chalmers' paper:

We might call this assumption a proportionality thesis: it holds that increases in intelligence (or increases of a certain sort) always lead to proportionate increases in the capacity to design intelligent systems. Perhaps the most promising way for an opponent to resist is to suggest that this thesis may fail. It might fail because here are upper limits in intelligence space, as with resistance to the last premise. It might fail because there are points of diminishing returns: perhaps beyond a certain point, a 10% increase in intelligence yields only a 5% increase at the next generation, which yields only a 2.5% increase at the next generation, and so on. It might fail because intelligence does not correlate well with design capacity: systems that are more intelligent need not be better designers. I will return to resistance of these sorts in section 4, under “structural obstacles”.