That part of synthetic biology concerned with engineering promises to make good on the potential of biotechnology to address problems of food, energy, health and environment. How do the synthetic biologists realize the promise of biology as technology? In analysing realization of promise in synthetic biology, I suggest that we should pay close attention to different rates of realization. Synthetic biologists have consistently focused on making particular kinds of devices such as oscillators, timers and clock that both address problems of control over rates, and that themselves resemble and link to other rate-controlling mechanisms such as the many clocks found in large technical systems. They have also, again in those parts of the field concerned with engineering, expended much effort in developing infrastructures, techniques, methods and systems for rapid assembly of parts and components. The clocks and assembly methods function as both as iconic signs and as infrastructural elements or practices that will realise the promise of biotechnology. The field has not only produced what we might call infrastructural-icons for biology as technology, but almost defined itself in terms of a promise of realisation. In analysing how synthetic biology or any other technological endeavour shows how things could be (icons), and makes operational connections between things (infrastructures), the main goal is not to situate field in social or economic contexts. Rather, it is to open a way to see how synthetic biologists and others -- philosophers, social scientists, historians, artists, designers, scientists engineers, as students or consumers–manage to address the gaps that open up as the promise of biology as technology is realized at different rates.