Maybe this doesn't answer your question, but I've always believed reading a book is not a race. In my experience, taking my time to understand, or re-reading, something has always helped me retaining knowledge.

I think trying to find ways to retain what you've read versus how fast you've read something will be more beneficial in the long run. In addition, by become better a reader and retaining what you've read, I believe your speed will increase naturally.

My issue was staying focused on what I've read without wandering off somewhere else in my brain. With patience and a little devotion, I was able to overcome that and find the true joy in reading.