brisk up vs speed

brisk up is not a noun while speed is a noun.

Word	Noun	Adjective	Verb	Adverb
brisk up	No	No	Yes	No
speed	Yes	No	Yes	No

As verbs, speed is a hypernym of brisk up; that is, speed is a word with a broader meaning than brisk up:

Other hypernyms of brisk up include accelerate, quicken, speed up.

brisk up (noun)	speed (noun)
	changing location rapidly
	a central nervous system stimulant that increases energy and decreases appetite; used to treat narcolepsy and some forms of depression
	a rate (usually rapid) at which something happens
	the ratio of the focal length to the diameter of a (camera) lens system
	distance travelled per unit time

brisk up (verb)	speed (verb)
become brisk	move faster
	cause to move faster
	move very fast
	travel at an excessive or illegal velocity
	move hurridly