time lag vs retardation

time lag and retardation both are nouns.

Word	Noun	Adjective	Verb	Adverb
time lag	Yes	No	No	No
retardation	Yes	No	No	No

As nouns, retardation is a hyponym of time lag; that is, retardation is a word with a more specific, narrower meaning than time lag:

Other hyponyms of time lag include extension, moratorium.

time lag (noun)	retardation (noun)
time during which some action is awaited	the act of slowing down or falling behind
	a decrease in rate of change
	lack of normal development of intellectual capacities
	any agent that retards or delays or hinders
	the extent to which something is delayed or held back