In the western US, going 3 mph under the speed limit would be considered a mortal sin.
But I do see the same effect going a mere 4 mph[1] over the speed limit.
1. I arrived at 4 mph over, because this was the usual answer when I've asked several police officers "How much over the speed limit to I have to drive for you to pull me over?". Their answer nearly always was "Up to 4 mph and you're okay."