precipitous
adjective
uk/prɪˈsɪpɪtəs/ usif a reduction is precipitous, it happens very suddenly and is very big:
a precipitous decline/drop/fall The company has seen a precipitous decline in advertising revenue.
done too quickly without thinking enough about any possible problems:
a precipitous decision
precipitously
adverb
Manufacturing has fallen precipitously.