fall vs slow down

fall is a noun but slow down is not a noun.

Word	Noun	Adjective	Verb	Adverb
fall	Yes	No	Yes	No
slow down	No	No	Yes	No

As verbs, slow down is a hyponym of fall; that is, slow down is a word with a more specific, narrower meaning than fall:

fall (noun)	slow down (noun)
a sudden drop from an upright position
the act of surrendering (usually under agreed conditions)
a lapse into sin; a loss of innocence or of chastity
a sudden sharp decrease in some quantity
a free and rapid descent by the force of gravity
a sudden decline in strength or number or importance
when a wrestler's shoulders are forced to the mat
a movement downward
a downward slope or bend
the season when the leaves fall from the trees
the time of day immediately following sunset

fall (verb)	slow down (verb)
occur at a specified time or place	become less tense, rest, or take one's ease
decrease in size, extent, or range	lose velocity; move more slowly
pass suddenly and passively into a state of body or mind	become slow or slower
be born, used chiefly of lambs	reduce the speed of
come out; issue	cause to proceed more slowly
assume a disappointed or sad expression
begin vigorously
come as if by falling
be cast down
go as if by falling
fall or flow in a certain way
move downward and lower, but not necessarily all the way
move in a specified direction
descend in free fall under the influence of gravity
slope downward
drop oneself to a lower or less erect position
lose an upright position suddenly
to be given by assignment or distribution
be captured
fall to somebody by assignment or lot; passed
come into the possession of
to be given by right or inheritance
be inherited by
lose office or power
yield to temptation or sin
suffer defeat, failure, or ruin
lose one's chastity
die, as in battle or in a hunt
be due
come under, be classified or included
touch or seem as if touching visually or audibly
fall from clouds