WordNet sense 1 (a noticeable deterioration in performance or quality):

drop-off, falloff, slump, falling off

WordNet sense 2 (a stretch of water without current or movement):

slack water

WordNet sense 3 (a soft wet area of low-lying land that sinks underfoot):

mire, morass, quagmire, quag

WordNet sense 4 (the condition of being loose (not taut)):

slackness

WordNet sense 5 (not tense or taut):

loose

WordNet sense 6 (not taut or rigid; not stretched or held tight):

lax

WordNet sense 7 (lacking in strength or firmness or resilience):

flaccid, limp

WordNet sense 8 (having little physical or spiritual strength):

weak

WordNet sense 9 ((of fluids) not moving or flowing):

standing

WordNet sense 10 (make less active or fast):

relax, slacken, slack up

WordNet sense 11 (become slow or slower):

slow, slow up

WordNet sense 12 (make less active or intense):

abate, slake

WordNet sense 13 (become less in amount or intensity):

let up, die away, slack off