Slacks up is a phrase that is generally used to express the idea of reducing or decreasing one's efforts, pace, or intensity. To explain the antonyms for the word slack up, it's necessary to consider its opposite, which is the expression 'picks up the pace.' It implies an increase in the pace, speed, or intensity of a person's actions or thoughts. Alternatively, other antonyms include intensify, heighten, amplify or accelerate, all implying the opposite of slack up by suggesting an increase in effort, energy, or activity. Employing these antonyms can help one remain focused and committed to achieving their goals or tasks without taking a break or slacking up.