Very fast; happening or developing quickly (e.g. rapid progress or change).
近年、技術の進歩は急速だ。
In recent years, technological progress has been rapid.
スマートフォンの普及は世界中で急速に進んだ。
The spread of smartphones progressed rapidly around the world.
急速な気温変化に体がついていかない。
My body can't keep up with the rapid temperature changes.