1
general::
Phrase(s): slow up
to go slower; to reduce speed in order for someone or something to catch up. • Slow up a little! I can’t keep up with you! • Please slow up. I can’t follow your lecture when you talk so fast., Phrase(s): slow someone or something up [and] slow someone or something down
to cause someone or something to reduce speed. • I’m in a hurry. Don’t try to slow me down. • Please slow up the train. There are sheep near the track.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Slacken or cause to slacken in speed, as in The train slowed up as it approached the curve, or Come on, you're slowing me up. [Late 1800s] Also see SLOW DOWN, def. 1.
American Heritage Idioms