The antonyms for the word speedup are slowdown, deceleration, retardation, delay, and decelerate. These words imply a decrease in the velocity, rate, or pace of something. A slowdown suggests a reduction of speed or activity; a deceleration denotes a negative acceleration, a slow-down in the speed of an object; a retardation indicates a deliberate slowing down; a delay suggests a postponement, a hindrance, or a pause; while decelerate refers to the act of gradually reducing speed. These antonyms are useful in describing scenarios where a decrease in speed or activity is required, such as in traffic, productivity, or work output.