slacken

(verb) become slow or slower. Synonyms: slow, slow down, slow up, slack. Example: "Production slowed."

(verb) make less active or fast. Synonyms: slack, slack up, relax. Examples: "He slackened his pace as he got tired." "Don't relax your efforts now."

(verb) become looser or slack. Example: "The rope slackened."

(verb) make slack as by lessening tension or firmness. Synonyms: remit.