On my Voyage and Paperwhite I always have the time left in chapter showing but I've never taken it too seriously. I don't think I read at a constant rate except, perhaps, when I'm timing myself.
I tend to re-read passages and look things up in wikipedia and the dictionary and stop to think about what I've just read. I doubt that the Kindle can possibly be accurate and I'm not sure I really care. If I see I have 2 minutes left in the chapter and I'm getting sleepy I'll get to the end before I put the book down. If I'm sleepy and it says 15 minutes I might just stop right then. That's really all it means to me.
Barry