Use the noun immediacy to talk about the lighting-fast speed with which something happens. If you order pizza and see the delivery driver pull up in front of your house two minutes later, you'll be amazed at the immediacy of your pizza's arrival.
Something that's immediate, or instantaneous, has the quality of immediacy. Another way to use this noun is to mean "directly," or "firsthand." For example, when people first viewed live news reports on television, they probably marveled at the immediacy of watching something as it was happening, directly and without delay. Immediacy has been in use since about 1600, and it comes from the adjective immediate, with its Latin root of immediatus, "without anything in between."