The Collaborative International Dictionary of English v.0.48:

Slowly \Slow"ly\, adv.
   In a slow manner; moderately; not rapidly; not early; not
   rashly; not readly; tardly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slowly
    adv 1: without speed (`slow' is sometimes used informally for
           `slowly'); "he spoke slowly"; "go easy here--the road is
           slippery"; "glaciers move tardily"; "please go slow so I
           can see the sights" [syn: slowly, slow, easy,
           tardily] [ant: apace, chop-chop, quickly,
           rapidly, speedily]
    2: in music; "Play this lento, please" [syn: lento, slowly]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "slowly":
   aridly, barrenly, bit by bit, bloodlessly, by degrees, by inches,
   by inchmeal, by slow degrees, cautiously, circumspectly,
   colorlessly, crawlingly, creepingly, degreewise, deliberately,
   dilatorily, dismally, draggily, drearily, drearisomely,
   drop by drop, dryly, dully, dustily, easily, effetely, emptily,
   falteringly, gently, gradatim, grade by grade, gradually,
   haltingly, heavily, hollowly, idly, in low gear, in march time,
   in slow motion, in slow tempo, inanely, inch by inch, inchmeal,
   indolently, insipidly, jejunely, lackadaisically, languidly,
   languorously, leisurely, lifelessly, limpingly, lingeringly,
   little by little, loiteringly, moderately, pallidly, ploddingly,
   pointlessly, pokily, pokingly, ponderously, relaxedly, reluctantly,
   slow, sluggishly, spiritlessly, step by step, stodgily, stuffily,
   superficially, tardily, tarryingly, tastelessly, tediously,
   tentatively, under easy sail, unhurriedly, uninterestingly,
   vapidly, with agonizing slowness, with deliberation