It took Darwin decades to write the Origin. He wrote it in all sorts of times, and actually was ill for a lot of it. Einstein did his seminal work in a non-plague time of peace (1905), though. Within one year published special relativity, his Nobel-winning paper on the photoelectric effect (proving the quantum nature of light), Brownian motion (finally proving the atomic theory of matter—yes, that was Einstein), and mass-energy equivalence. Busy year, better example.