It's best if people all move at more or less the same speed. It keeps them better spaced. People driving much slower than that can cause as many difficulties as people driving much faster.
We recognize the dangers of driving too fast, and most people try to keep it to near the speed limit, at least as long as the limit is set properly. Some are set very badly, and that's hazardous. You get a mix of people traveling at a safe but illegal speed with people obeying the law.
Fortunately, I've found that most speed limits aren't too badly off. I'm sure there are jurisdictions where they're deliberately mis-setting them as revenue generators, but I don't encounter many of them. (I can name one not too far from my house, where a four-lane divided road with minimal access has a 30 MPH speed limit... and a speed camera on a big downhill leg. That's going to get people killed, because everybody who knows about the speed limit jams on their brakes and goes 25. And the road is a major arterial, or it could be, if they didn't deliberately limit the flow rate so badly. The road is, of course, a nightmare at rush hour and a speed-trap revenue source the rest of the time.)