The best speed is dependent on the stiffness of your springs mostly. I grew up in the country with those washboard roads. With a normal US sedan, 30 mph was a livable speed, although 50 mph gave a smoother ride, but that would be 20 mph over the speed limit.
Here in the mountains we have open range grazing, and there are many “Texas gates” which are gaps in the road filled with round pipes far enough apart to discourage cattle from crossing them. There are narrow strips of solid steel to drive over with the average car. Small compacts with narrow treads have to take the bumps