I've been wondering where my Track Your Reading Speed in Kindle had disappeared to while reading/listening on my Kindle Fire (5th gen). I just found the solution and it's so simple that if yours doesn't work it's entirely possible you don't even remember doing it.
The link above has the solution and it's so simple. While your kindle is on and you're on a book page simply touch the bottom left corner of the book screen and it will toggle from blank to reading speed to location. Remember, it's not the same as if you were touching on the middle of the screen to show the header and footer menu bars, just a reading page.
So glad I found the solution.