WordNet (r) 3.0 (2006):

slow up
    v 1: become slow or slower; "Production slowed" [syn: slow,
         slow down, slow up, slack, slacken]
    2: cause to proceed more slowly; "The illness slowed him down"
       [syn: slow, slow down, slow up]
    3: lose velocity; move more slowly; "The car decelerated" [syn:
       decelerate, slow, slow down, slow up, retard] [ant:
       accelerate, quicken, speed, speed up]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "slow up":
   afterthought, arrest, backpedal, backwater, bind, block, blockage,
   brake, bureaucratic delay, check, clip the wings, curb, decelerate,
   deceleration, delay, delayage, delayed reaction, detain, detention,
   double take, drag, dragging, draw rein, ease off, ease up,
   ease-off, ease-up, flagging, halt, hang-up, hindrance, hold back,
   hold in check, hold up, holdup, impede, interim, jam, keep back,
   lag, lagging, let down, let up, letdown, letup, logjam,
   lose ground, lose momentum, lose speed, minus acceleration,
   moderate, moratorium, obstruct, obstruction, paperasserie, pause,
   red tape, red-tapeism, red-tapery, reef, rein in, relax, reprieve,
   respite, retard, retardance, retardation, retardment, set back,
   setback, slack, slack off, slack up, slack-up, slacken, slackening,
   slow, slow down, slowdown, slowing, slowing down, slowness, slowup,
   stay, stay of execution, stop, stoppage, suspension, take in sail,
   throttle down, tie-up, time lag, wait