slacken

/ˈsla-kən/

verb

to make less active : slow, up
to make slack (as by lessening tension or firmness)
to become slack or slow or negligent : slow, down

slacken speed at a crossing

delay retard slow slacken detain mean to cause to be late or behind in movement or progress. delay implies a holding back, usually by interference, from completion or arrival. retard suggests reduction of speed without actual stopping.