let up vs slacken off

let up

verb
  • To cease or stop (doing something intense). 

  • To cease or stop. 

slacken off

verb
  • To do (something) less intensely. 

  • To slacken or lessen; to become less intense. 

How often have the words let up and slacken off occurred in a corpus of books? (source: Google Ngram Viewer )