In one word, leadership. A senior anything is someone with enough experience to be able to lead a development effort. They should have a proper understanding o

In one word, leadership. A senior anything is someone with enough experience to be able to lead a development effort. They should have a proper understanding of what it takes to build a reliable and robust system end to end. "How quickly" is the wrong question. It's not about speed of delivery, but quality. A senior engineer would know that and if someone asks them to build something quick, they should be honest about the loss in quality and technical debt that will be incurred due to speed of delivery.