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 fall to under a hundred pounds"; "his voice fell to a whisper"

slow up, slow, slow down - cause to proceed more slowly; "The illness slowed him down"

delay, detain, 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"

modify, qualify - add a modifier to a constituent

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"