Quick-acting

Beginning to produce its intended effect after little delay.

Reach for it when describing onset speed, especially for a remedy or response

A quick-acting valve stopped the water before it reached the archive.

• everyday

  fast-acting

  rapid onset is the simple claim

  not when Fast-acting can sound promotional without evidence.

• everyday

  immediate

  there is effectively no perceptible delay

  not when Immediate is a stronger timing promise.

• professional

  responsive

  the system reacts quickly to input

  not when Responsive may not complete the intended action.

• formal

  prompt

  timely response matters more than mechanism

  not when Prompt describes readiness as well as speed.

• formal

  rapid

  measured speed is already clear from context

  not when Rapid may describe the whole process, not onset.

• literary

  swift

  lively praise suits a narrative

  not when Swift is less precise for medical claims.

• slow-acting
• delayed
• inert

• quick-acting remedy

  The pharmacist recommended a quick-acting remedy for temporary relief.

• quick-acting response

  A quick-acting response team contained the spill.

• quick-acting mechanism

  The quick-acting mechanism released the safety brake.