Read what’s comfortable. If you subvocalize, it’s totally fine. If you try to think about how fast you’re reading you’re more likely to end up missing the content because you’re actively detracting away from what you’re reading.
I wouldn’t worry about it too much, but if you insist on trying to wear out the idea, I would try practicing with books you would like to re-read. Familiarity with the content will make it easier to re-learn what you’re taking in.
Not everybody has to enjoy the same thing; though she does seem stubborn to try considering that her argument that imagination is hard work seems silly. But it’s possible she doesn’t draw mental pictures well in her head and consequently that is where she is coming from. There is possibly a level of doubt she has for herself because she doesn’t “get” the visuals books describe.
One thing you could try is to incorporate reading as a “together” activity where you two may take turns reading a book aloud to each other. By making it something you and her can do together then perhaps you can engage her both in books and your relationship that way.
Tangent to books, broaching this subject can sometimes come across as a personal attack on people since they is an attitude being a bookworm can possess for some people. Sometimes trying to encourage reading can appear as though you are subjecting your “superior” hobby to someone because they are “dumb”. Just be mindful how you engage her in the discussion.
They use the word divergent to explain as being unique and special. From the perspective of those trying to fix human genetics, a divergent is basically a step in resolving the genetic mix needed. I think the point to not mention you were divergent to live a “normal” life in the society until generations’ worth of divergent mixing normalized the majority of the populace.
A lot of people hated on the third book, but it felt like to me it was the most “whole” of a book because you got a much needed explanation on the world-building.