The antonyms for the phrase "slackening pace" can be energetic, swift, brisk, or rapid. These words suggest a sense of urgency and quickness in action, rather than relaxation or laziness. An energetic pace implies a dynamism and enthusiasm to complete tasks. A swift pace indicates a high speed of movement, and a brisk pace denotes a speedy movement while maintaining a healthy rhythm. Rapid pace suggests an accelerated rate of completion of tasks. Thus, instead of a slackening pace, one can adopt an energetic, swift, brisk, or rapid speed to remain productive and proactive.