decrease,
diminish,
lessen,
fall decrease in size, extent, or range; "The amount of homework decreased towards the end of the semester"; "The cabin pressure fell dramatically"; "her weight fell to under a hundred pounds"; "his voice fell to a whisper"
detain,
delay,
hold up cause to be slowed down or delayed; "Traffic was delayed by the bad weather"; "she delayed the work that she didn't want to perform"
decelerate -
reduce the speed of; "He slowed down the car"
modify,
alter,
change make less severe or harsh or extreme; "please modify this letter to make it more polite"; "he modified his views on same-gender marriage"
moderate make less fast or intense; "moderate your speed"
retard cause to move more slowly or operate at a slower rate; "This drug will retard your heart rate"
fishtail slow down by moving the tail sideways; "The airplane fishtailed on the runway"