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 lowrent 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.
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.