I think there's a point at which "reading rate" conflicts with "reading enjoyment" and/or "reading comprehension." When looking for a example of doing life wrong, as always, Elon Musk provides: He recently tweeted a much-mocked picture of The Odyssey in a tweet talking about how everyone should read The Iliad. While that was what most folks focused on, he also suggested an audiobook of The Iliad at 1.25 speed. Any anyone suggesting a book of poetry, of all things, be read at 1.25 speed is someone who clearly thinks the only goal is to say that they've finished a book, not to actually enjoy it.

Obviously, practice does improve one's rate, but in the end, it's carving aside more time to read that's really what's needed. I read when I wake up, before I go to bed, when I'm commuting to work (hopefully will be something that happens again soon), at meals, and also carve out time to read during days and evenings (having a wife who also likes to read helps). In most cases, all I'm sacrificing is time I'd have spent playing phone games or the like.

I also do what you do and have multiple books going on at once -- sometimes, I'm just not engaging with nonfiction or sci-fi, so a mystery is good to have on hand, etc.