quick on the draw
Also, quick on the trigger. Rapid in acting or reacting, as in You have to be quick on the draw if you want to find low-rent housing here, or Bruce was quick on the trigger when it came to answering questions. The first expression came from the American West's gunslingers and was broadened to mean “a quick reaction” in the first half of the 1900s. The variant originated about 1800.