I was out last night with my D5100 taking 20 second exposures of cars driving past my house. Why did each exposure take about 20 seconds to write to the memory card? It seems that with most of the picture black, there would be less information that had to be written.
#2. "RE: Why long exposure long to write" In response to Reply # 0
You probably have Long Exposure Noise Reduction turned on. This feature is used to reduce the amount of digital noise in the image. It has to make a second exposure of equal length to the "real" exposure and then do some processing, so that is what you have to wait for. The image itself takes no longer than normal to write to the memory card.