instantly vs straightaway

instantly is not a noun while straightaway is a noun.

Word	Noun	Adjective	Verb	Adverb
instantly	No	No	No	Yes
straightaway	Yes	Yes	No	Yes

As adverbs, instantly and straightaway are synonyms defined as:

instantly and straightaway: without delay or hesitation; with no time intervening

Other synonyms of instantly include at once, directly, forthwith, immediately, like a shot, now, right away, straight off.

instantly (noun)	straightaway (noun)
	a straight segment of a roadway or racecourse

instantly (adjective)	straightaway (adjective)
	performed with little or no delay

instantly (adverb)	straightaway (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening
without any delay