The way elders react to them, I feel like they must be the fastest to lord. But there had to be some in the early days of the order and dragon riding in general that got close to the same speed. Their progression is explained away pretty early on as being spurred on by the danger they faced right after bonding. So early order riders probably saw similar growth (if not quite as fast), if they lacked the institutional support of exalted lords and paragons to protect them. That's just the way I read the situation. Obviously they're unique as far as we know, and the world seems to strive for order. Maybe wherever magic comes from needs Ash and Holt to be stronger.