The word most slackened implies a decrease in tension, work or effort. Therefore, antonyms for most slackened would suggest an increase in tension, activity or concentration. Words such as strengthened, intensified, tightened, accelerated, aroused or heightened could be used as antonyms for most slackened. These words suggest a heightened state of energy, effort or tension. The antonyms for most slackened can be used to describe situations where there is an increase in activity or tension. For instance, "the team intensified their training regime" or "he tightened his grip on the steering wheel" suggests an increase in effort and concentration.