slow up in WordNet English dictionary

1. lose velocity; move more slowly
   "The car decelerated"
   decelerate  slow  slow down  slow up  retard 
   
2. become slow or slower
   "Production slowed"
   slow  slow down  slow up  slack  slacken 
   
3. cause to proceed more slowly
   "The illness slowed him down"
   slow  slow down  slow up 
   

slow up in English Wiktionary