A slower speed is dangerous to everyone. One car slows down and everyone else needs to get around them. That results in people who would otherwise be content to flow with traffic making aggressive moves to pass the obstruction. These actions catches some people in the other lanes unaware and some of them insensitively hit their brakes (kind of like the people who brake for curves at low speed because "it's what you do") which exacerbates the problem.

Now add in a poorly marked road dense traffic and construction. Other drivers (as a ground) don't really have the spare attention to allocate to the fallout of someone who's slowing down.

edit: This is backed up by traffic studies When everyone is driving the same speed things are safer because there's less "friction" (I think that's the correct term) between traffic elements.