Mike: regarding the quoted section of your post, AFAICT this ONLY happens when you pass near a heavenly body. I have traveled 500-300,000 Ls hundreds of times without that occurring.
relayer: I actually don't think that was exactly the intention of the devs, I think it was intended to tell you when approaching a body (as a target destination) that you are going too fast, to "slow down." I personally think it should say "too fast for approach." Unfortunately, as evident by the exact mechanics in the game, it seems to be a bit artificial in nature, and to me it seems like the devs are just trying to punish you for using too much throttle on approach. I have had times where I accidentally changed my target during this event and I immediately slowed down, but I've tried since and can't reproduce that. Also, the effect seems to be exactly the same in intensity and range (when there are no other bodies within affected range, not counting the body the target is orbiting) no matter what the gravity level of the body is, whether it is a huge planet, star, moon, orbital space station (or even a USS?); which doesn't make any sense, which is big argument for the notion that it is just arbitrarily added by the devs to punish you for approaching at excessive throttle.
Your ship will slow down as you approach objects of sufficient mass, stars, moons, planets, and what not. The slow down warning is thrown when your ship believes that you are attempting to approach whatever it is you are close to. You can see the same thing when you are trying to drop at a station or a nav beacon and are going too fast. The best explanation I have heard that it is a combination of any applicable gravity wells and your ship auto decelerating. Well, not actually decelerating due to how FSDs work but you get the idea. If you watch your throttle indicator, you will see that both the throttle setting indicator (the line that moves when you use the throttle) and the blue zone will shift positions up and down based on your proximity to large objects and your destination. However, despite those moving, your actual throttle setting doesn't change. If you are at 100% throttle, then even if the throttle setting line is at the bottom end of the range, you can't throttle up. It is also possible for your speed indicator to be higher than where the throttle indicator is, indicating that your ship is "slowing down," for lack of a better way to put it. Given how FSDs work, in that they move space around your ship, this is probably a safety feature so that the FSD doesn't try to move a whole planet around your ship. Can you imagine the consequences? A basic rule of physics at all levels is that two objects cannot occupy the same space at the same time.