WordNet (r) 3.0 (2006):

slowdown
    n 1: the act of slowing down or falling behind [syn: slowdown,
         lag, retardation]

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "slowdown":
   action, afterthought, arrest, bind, block, blockage, boom,
   bottoming out, boycott, boycottage, bureaucratic delay,
   business cycle, business fluctuations, bust, catabasis, check,
   cheeseparing, collapse, cooling, cooling down, cooling off, crash,
   crisis, curtailment, cutback, deceleration, declension, decline,
   decline and fall, decrescendo, delay, delayage, delayed reaction,
   depression, detention, diminuendo, dive, double take, downtrend,
   downturn, drag, dragging, drop, dwindling, ease-off, ease-up, ebb,
   economic cycle, economic expansion, economic growth, economization,
   economizing, expanding economy, expansion, fall, falloff, flagging,
   freeze, growth, halt, hang-up, high growth rate, hindrance, holdup,
   inactivity, interim, jam, job action, lag, lagging, lapse, letdown,
   letup, lockout, logjam, low, low growth rate, market expansion,
   minus acceleration, moratorium, obstruction, outlaw strike,
   paperasserie, pause, peak, peaking, plunge, prosperity, protest,
   recession, recovery, red tape, red-tapeism, red-tapery,
   reduction of expenses, reduction of spending, remission, reprieve,
   respite, retardance, retardation, retardment, retreat,
   retrenchment, revolt, rulebook slowdown, saving, scraping,
   scrimping, setback, sick-in, sit-down, sit-down strike, skimping,
   slack, slack-up, slackening, slow-up, slowing, slowing down,
   slowness, slowup, slump, sparing, stagnation, stay,
   stay of execution, stop, stoppage, strike, subsidence, suspension,
   sympathy strike, tie-up, time lag, turnout, upturn, wait, walkout,
   wane, wildcat strike, work stoppage