straightaway vs instantly

straightaway is a noun but instantly is not a noun.

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

As adverbs, straightaway and instantly are synonyms defined as:

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

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

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

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

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