drop off vs slacken off

drop off

verb
  • To lessen or reduce. 

  • To fall. 

  • To deliver; to deposit or leave; to allow passengers to alight. 

  • To fall asleep. 

slacken off

verb
  • To slacken or lessen; to become less intense. 

  • To do (something) less intensely. 

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