modularity requires cognitive efficiency
First, what do I mean by cognitive efficiency.
Some tasks require mental effort.
For example, suppose I ask you to count from one to one hundred omitting each prime number.
This is not the kind of thing most people can do while washing up.
It requires attention, inhibitory control and working memory.
By contrast, many adults can count in the ordinary way from one to one hundred while washing up.
Counting has become routine, habitual.
So much for cognitive efficiency.
Now why suppose that modularity (or core knowledge) requires cognitive efficiency?
To invoke modularity, we need to understand how mindreading could be (i) automatic and (ii) present in pre-linguistic infants with limited working memory & executive function; both (i) and (ii) mean we need to understand how it could be cognitively efficient. (The situation is a bit like this: we want to say you can perceive others mental states; but on the face of it, mental states are exactly the sort of things that are not available to perception.)
How could mindreading ever (but not always) be automatic?
Representing perceptions and beliefs as such---and even merely holding in mind what another believes, where no inference is required---involves a measurable processing cost\citep{apperly:2008_back,apperly:2010_limits}, consumes attention and working memory in fully competent adults,\citealp{Apperly:2009cc, lin:2010_reflexively, McKinnon:2007rr} may require inhibition\citep{bull:2008_role} and makes demands on executive function.\citep{apperly:2004_frontal,samson:2005_seeing}