As a parent of 4 that has homeschooled all of them at one point or another, I could relate a lot to this with respect to my youngest, who is 6, and reading a year or two ahead of his age level (and who just achieved 3rd place in a state-wide young writers contest). He (a) storms ahead glossing over words sometimes making wrong guesses, and (b) resents it if fed answers two quickly when he does slow down to try to figure out a word.
I have a theory that he is a potential speed reader (which I am not). There are times when I just let him blaze ahead uninterrupted, and other times when I'll gently correct him, sometimes after he's completed the sentence. I'm afraid that knowing when to do which is partly a matter of intuition, but if he seems excited and anxious to see what comes next, that's a clue this isn't a good time to slow him down.
As for the times when he does slow down and try to figure out a word, I wait about 3 seconds or until he looks up or asks for help. If he's working on it, I find it's better to err in the direction of giving him plenty of time.
In my opinion, there are two modes of reading, speed reading to get the overall idea, and careful/analytical reading such as used for (a) learning a difficult subject, (b) reading aloud to others, or (c) proofreading. It is good to learn both. I am thinking an 8 year old can probably understand that sometimes it's ok to go fast, but sometimes you need to read carefully. If the two of you can agree on that, then you can decide together if a given time is a "fast" time or a "careful" time.
Again, I'm not a recognized expert, just the child of two teachers who has children who are now 32 (with Asperger Syndrome, still working his way through college), 30 (with ADD, just completed a 2 year degree after 3 years in the Army), 20 (home-schooled by me throughout high school, and now a 4th year art major), and 6 (home-schooled by me for the first 3 months of first grade after a disastrous beginning, followed by a successful reintegration). My wife and I also watch infants/preschoolers as a family child care home.