slow vs slow up

slow is an adjective but slow up is not an adjective.

Word	Noun	Adjective	Verb	Adverb
slow	No	Yes	Yes	Yes
slow up	No	No	Yes	No

As verbs, slow and slow up are synonyms defined as:

Other synonyms of slow include decelerate, retard, slow down.

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

Other hyponyms of slow include slow, slow down, delay, detain, hold up.

As verbs, slow and slow up are synonyms defined as:

Other synonyms of slow include slack, slacken, slow down.

As verbs, slow and slow up are synonyms defined as:

Other synonyms of slow include slow down.

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

Other hypernyms of slow include decelerate, retard, slow, slow down.

slow (adjective)	slow up (adjective)
not moving quickly; taking a comparatively long time
at a slow tempo
(used of timepieces) indicating a time earlier than the correct time
(of business) not active or brisk
slow to learn or understand; lacking intellectual acuity
so lacking in interest as to cause mental weariness

slow (verb)	slow up (verb)
lose velocity; move more slowly	lose velocity; move more slowly
become slow or slower	cause to proceed more slowly
cause to proceed more slowly	become slow or slower

slow (adverb)	slow up (adverb)
without speed (`slow' is sometimes used informally for `slowly')
showing a time that is earlier than the actual time