Oh, how I do love a thoroughly counter-intuitive discovery! Apparently, the plainer and cleaner the typeface, the less a reader will learn and remember of the detail of the text. A typeface which slows the reader means they learn and understand more of what's being said. Not just the denotation, but the connotations, the friction between them, the prosody which affects the tone and 'feel' of the piece... they all have time to grow and flower, and create a full meaning, rather than only a basic meaning, in the reader's mind.
This sounds to me like something fairly fundamental in human consciousness. In Rembrandt's painting The Jewish Bride, for example, the huge, thickly embroidered sleeve of the man is the most extraordinary assemblage of paint, whereas other areas are quite smoothly painted to convey basic information. Rembrandt used the skin from paint left out overnight to create the the white highlights, and even as you sense the richness of the cloth and the pride of the man you can see the wrinkles and bumps of the paint; your eye-movements catch on the physical nature of the art, the extreme paint-ishness, in a way which makes the mimetic nature of art – the evocation of the garment – even more vivid. The 17th century treatises on painting that Rembrandt knew said the same: rough paint holds the eye longer than smooth, they declared. And I don't think you have to know anything about painting technique, let alone be thinking about it, for that effect to work on you; it's built into what the artist has done to the canvas.
Journalists are taught to work with received phrases - smoothly painted, if you like - so that readers can understand them with minimum puzzlement and maximum speed, and move on. Whereas one important characteristic of literary fiction is almost always that the words on the page are more unusual for that subject, or more unusually put together. Once un-received words or conjunctions of words catch on the ear and mind as the eye catches on the roughness of the sleeve, then the mind has to work harder – so probably slower – to understand it. This frustrates readers who just want to get on with the story, but at a slower speed of taking in the denotation, the connotations to begin to flower. (Though readers who don't know how to slow down deliberately in reading an apparently super-simple text may also be frustrated by not finding those flowers either.) That's not about consciously appreciating technique in a writerly way, versus immersing in the story, as much pro- and anti-literary talk assumes; that's about how the brain works.
Experts in pedagogy call it desirable difficulty. Studies show that students are most satisfied by the courses where both goals and processes are made very clear, and are worked through in an organised way. But they also show that such courses don't help students gain good understanding or grades nearly as well as courses where both processes and goals are more opaque and less easily found. In having to work out what they need from the course, rather than being told, and then grapple it out from what they're offered instead of being shown what they must learn, students integrate the new knowledge and understanding: it becomes their own.
And then a student learning to write poetry said how hard last lines seemed to be: it was difficult to avoid them being trite or a let-down. And I talked about the first of Edwin Morgan's Glasgow Sonnets, and said about the end,
What he hasn't done is gone on to write - to spell out, make explicit - the conclusion that he (presumably) wants you to come to. The poem led us to that point, and now leaves us there, to think and feel in the space after the last line; the understanding is ours, not handed to us by the poet...
As any adult finding themselves crying or laughing on cue in a well-made Disney film knows, we can be made to think and feel precisely as an artist wants us to. But the experience of most really good art is a more ambiguous business; it offers both sleeve and paint and the perceptual friction between them, to get you to grapple out what you see, think and feel. And because it's you who did the grappling, the thought and feeling are yours: you make the book, or the picture, your own.