instantly vs forthwith

instantly and forthwith both are adverbs.

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

As adverbs, instantly and forthwith are synonyms defined as:

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

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

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