Difference between Slow Down and Slacken

SLOW DOWN vs SLACKEN: NOUN

A spongy, semivitrifled substance which miners or smelters mix with the ores of metals to prevent their fusion.
The slags or cinder from previous fusions, used in smelting operations to mix with natural ores and to retard fusion of the ores until reduction shall have proceeded to the desired point.

To become less active; fall off: as, trade slackened; the demand slackens; prices slacken
To become remiss or neglectful, as of duty.
To make slack or slacker.
To abate; moderate; lessen; diminish the intensity, severity, rate, etc., of; hence, to mitigate; assuage; relieve: as, to slacken one's pace; to slacken cares.
To be or become remiss in or neglectful of; remit; relax: as, to slacken labor or exertion.
To become slack.
(transitive; intransitive verb) To make or become less vigorous, intense, or severe; ease.
(transitive; intransitive verb) To make or become less tense, taut, or firm; loosen.
(transitive; intransitive verb) To make or become slower; slow down.

Back off, Speed limit, Speed up, Slow up, Unlax, Slack, Slacken, Unwind, Decompress, Unbend, Decelerate, Relax, Loosen up, Slow, Retard

Letup, Relax, Diminish, Loosen, Abate, Waver, Relent, Weaken, Wane, Slack up, Slow up, Slow down, Remit, Slack, Slow

Stay off, Go back, Catch up, Back off, Speed up, Slow up, Slack, Slacken, Decompress, Unbend, Decelerate, Relax, Loosen up, Slow, Retard

Detach, Lessen, Decrease, Loose, Letup, Relax, Diminish, Loosen, Abate, Weaken, Slow up, Slow down, Remit, Slack, Slow

Slow down and shift down to a speed that you can controlwithout using the brakes hard.
Time Stop: Slow down the passage of time so slow it eventually freezes, stunning all enemies in the area.
It is likely that traffic will slow down, or even stop, further down the road.
Fortunately, you can combat the risk of bloat by helping your Labrador slow down their rate of eating with a slow feeder.
OTR and yes things do slow down a bit after the holidays but at least when I was there they were really never slow.
Slow debuff, which can greatly slow down a player, even if just for a second.
Does it force you to sit down, lie down, slow down?
The problem is so severe that it will even slow down my internet feed and slow down other excel files I have open.
They gradually slow down, forcing pursued vehicle to slow down by allowing no outlet.
When i slow down to turn a corner, or slow down for a traffic light, going at a slow speed from approx.