For fiction, I just don't enjoy it if I read quickly. For sf, I like to ponder the ideas. For non-sf, I like to ponder the relationships. Holy cow--if we ever had to share one book simultaneously, we'd drive each other insane! I rush through so I can find out what happens! Then, if I liked the book well enough, I might go back and find a detail or two that I missed. As to relationships--the only time I bother with extra time on those is if the book has a great number of characters in it--and then only if the book seems worth my while to do so. I do love re-reading the really good books, though.