[1] : WordNet (r) 2.0

slow up
     v 1: become slow or slower; "production slowed" syn: slow, slow
          down, slack, slacken
     2: cause to proceed more slowly; "the illness slowed him down"
        syn: slow, slow down
     3: lose velocity; move more slowly; "the car decelerated" syn:
         decelerate, slow, slow down, retard ant: accelerate
see also:
slow slow down slack slacken slow down decelerate 
retard accelerate 

[2] : 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

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